No, they can't be enchanted, because they aren't masterwork and can never be, and all magic weapons must be crafted with a masterwork base.
Creating Magic Weapons
To create a magic weapon, a character needs a heat source and some iron, wood, or leatherworking tools. She also needs a supply of materials, the most obvious being the weapon or the pieces of the weapon to be assembled. Only a masterwork weapon can become a magic weapon, and the masterwork cost is added to the total cost to determine final market value.
It is quite plain that they are not masterwork by default, given the lack of +1 to attack rolls. So let's suppose we want to upgrade them. But it is impossible for an existing unarmed strike to be made masterwork, per this clause in Weapons:
You can’t add the masterwork quality to a weapon after it is created; it must be crafted as a masterwork weapon (see the Craft skill).
All right, let's try making a new one that's masterwork. This fails because unarmed strikes are not manufactured weapons, and you can't manufacture a weapon with the Craft skill if it's specifically not manufactured at all.
Glossary — manufactured weapons:
This category also includes […] in essence, any weapon that is not intrinsic to the creature.
Monk's class features do not help with this, since Craft is not an effect and neither enhances nor improves anything:
A monk’s unarmed strike is treated both as a manufactured weapon and a natural weapon for the purpose of spells and effects that enhance or improve either manufactured weapons or natural weapons.
How about buying one? You might actually be able to buy an ordinary unarmed strike, since they're listed in the Equipment section in the Weapons table, and you needn't pay anything for it*. Unfortunately, per the quote on MW weapons, a weapon must be crafted as masterwork to be MW, and per the preceding this is not possible even for NPCs. There might be an exception, as there seems to be for magic oils, which you can buy in shops but not craft, except that nowhere is "MW unarmed strike" listed for sale.
Magic fang and company do not make a weapon into a "magic weapon"; rather, they give a weapon a +1 enhancement bonus (and the ability to bypass DR /magic because of that), which is not technically the same thing. And while all magic weapons are masterwork by definition as well as by prerequisite, magic fang and friends do not have to be interpreted as giving a weapon retroactive masterwork status, for essentially the same reason, which is fortunate as otherwise the rules would create some bizarre and abusive situations†.
Magic Fang
Magic fang gives one natural weapon of the subject a +1 enhancement bonus on attack and damage rolls.
Magic Weapon
Magic weapon gives a weapon a +1 enhancement bonus on attack and damage rolls. (An enhancement bonus does not stack with a masterwork weapon’s +1 bonus on attack rolls.)
You can’t cast this spell on a natural weapon, such as an unarmed strike (instead, see magic fang). A monk’s unarmed strike is considered a weapon, and thus it can be enhanced by this spell.
Magic Weapons
All magic weapons are also masterwork weapons, but their masterwork bonus on attack rolls does not stack with their enhancement bonus on attack rolls.
Special Abilities
Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters.
Note the phrasing of the last quote; they are described as vulnerable to magic weapons, but that's not the rules text, just the plain description, since vulnerability is not defined (except in the incompatible context of energy vulnerability; it should be clear that DR 5/magic does not mean that a +1 longsword does 50% extra damage as well as overcoming DR). The actual rules say that any weapon with a +1 magical enhancement bonus (whether or not the weapon is, per se, a "magic weapon" itself otherwise) overcomes that DR.
(emphasis added to quotes)
* This is, to my mind, patently absurd, but hey, RAW. Monks won't be proficient with those unarmed strikes, but hey, RAW.
† For example, casting magic weapon to increase the price of a club from 0gp to 300gp (permanently? who knows!) and selling it. Or casting magic weapon on any plain weapon to make it masterwork for future crafting. There is a Pathfinder spell for this purpose. It does nothing else‡, is a level higher, costs as much as crafting a MW weapon in the first place, and takes an hour to cast. Allowing magic weapon to do this for free in a standard action is ridiculous.
‡ It has this excellent provision: "If the target object has no masterwork equivalent, the spell has no effect." Doesn't work on unarmed strikes, as they appear to have no MW equivalent and are not objects.
Attack roll for unarmed strikes = d20 + proficiency bonus + Strength modifier.
Unarmed strikes are, in fact, melee weapon attacks - even though they're not made with a weapon. This is clarified in the Sage Advice Compendium:
What does “melee weapon attack” mean: a melee attack with a weapon or an attack with a melee weapon?
It means a melee attack with a weapon. Similarly, “ranged weapon attack” means a ranged attack with a weapon. Some attacks count as a melee or ranged weapon attack even if a weapon isn’t involved, as specified in the text of those attacks. For example, an unarmed strike counts as a melee weapon attack, even though the attacker’s body isn’t considered a weapon.
Here’s a bit of wording minutia: we would write “melee-weapon attack” (with a hyphen) if we meant an attack with a melee weapon.
This is also supported by the fact that the rules on unarmed strikes are stated in the the "Melee Attacks" section of the rules; the wording is simply clarifying that if you want to make a melee weapon attack without a weapon, you can use an unarmed strike to do so.
As such, the attack roll is calculated as follows:
- d20 + proficiency bonus + Strength modifier
Best Answer
This would not be imbalancing and only affects a small-ish (5-29) number of specific things
Unarmed strikes already activate and qualify for various features due to them counting as "attacks", "melee attacks", "weapon attacks", and "melee weapon attacks". As shown in this Q/A the only things unarmed strikes currently cannot qualify for are attacks which require you to use weapons (also ranged attacks of any kind, but making unarmed strikes count as weapons would not change that fact).
Notably, unarmed strikes cannot be ranged and do less damage than any other weapons, this means that allowing them when a feature requires "an attack with a weapon" would not increase the versatility of these features. One situation this change would help with is if you had no weapons available (such as after being capture) you would now be able to use an unarmed strike to activate these features; the only features which this affects meaningfully are divine smites and the blade cantrips as they could be used even when completely without weapons adding a sizeable amount of damage. However scenarios where you have absolutely no weapons available are situational and few and far between so I would not call them imbalancing.
Additionally there are features which already improve unarmed strikes (such as the Monk's martial arts dice) and combining these with unarmed strikes counting as weapons would not create significant imbalance, especially since every Monk feature that allows Monk Weapons to be used also allows unarmed strike (there is one exception discussed later).
Having unarmed strikes count as weapons gives very slight buffs to features in specific situations but these are rare and small enough that they are not imbalancing.
I will now provide a full list of the effects this change would have (well, a list of all the things I could find):
The features affected
The Paladin’s Improved Divine Smite feature states:
So you could use improved divine smite with an unarmed strike but this would only help you if you were weren't wielding any weapons at the time, or were facing a for you would not want to use weapons against such as those that corrode metal weapons.
Extremely technically the Crown Paladin’s Exalted Champion feature states:
Normally unarmed strikes are not weapons, and so they would bypass this resistance; however, this same phrase existed in the Monster Manual and a few places in the Player’s Handbook and errata have since changed instances of “nonmagical weapons” to “nonmagical attacks”. This is also shown in this Q/A.
It is likely this usage for the Crown Paladin was simply not noticed, and that it should say “nonmagical attacks”. So technically, if you read the feature incredibly strictly, counting unarmed strikes as weapons would change the feature.
The enlarge/reduce spell states:
And for the reduction option the opposite occurs. Normally this change to damage only affects weapons and so not unarmed strikes, but now it would affect them as well, this change is small but would benefit Monks using Flurry of Blows and similar features.
There is also the Great Weapon Master feat which states:
Normally this cannot apply to unarmed strikes as they are not weapons, but with the change it could.
Similarly the two blade cantrips (green-flame blade and booming blade) say this:
So they would usually not work with unarmed strikes, but with this change they would, again this change only has much of an affect if you had no weapons available.
Can you touch or wield an unarmed strike?
There are some features which make having unarmed strikes count as weapons somewhat confusing:
There are five similar features: The Forge Cleric's Blessing of the Forge feature, the Hexblade Warlock’s Hex Warrior feature, the magic weapon spell, the elemental weapon spell, and the holy weapon spell. All of these have similar descriptions effectively containing the phrase:
Assuming you can touch an unarmed strike, you would now be able to use these affects on them (what this means is not clear and is not laid out in the rules elsewhere so it would be up to the GM to determine how this change would work).
There is also the Devotion Paladin’s Channel Divinity: Sacred Weapon feature states:
Similarly there is the Warlock’s Pact of the Blade feature which states:
Thus they could create unarmed strikes (what this means is up to the GM) which would gain the usual benefits of summoned weapons such as overcoming resistance and immunity to nonmagical attacks and damage.
All of the Warlock’s Eldritch Invocations which usually only work with pact weapons would now potentially also work with unarmed strikes.
These are the Lifedrinker Invocation which states:
The Thirsting Blade Invocation which states:
The Eldritch Smite Invocation which states:
And the Improved Pact Weapon Invocation which states:
Though what using an unarmed strike as a spellcasting focus would mean it up to a GM.
Finally the Dueling Fighting Style states:
This would likely not need a ruling as you can’t really wield an unarmed strike, but it’s an interesting case nonetheless.
Remember the above section of changes is all from the confusing idea of summoning, touching, or interacting with an unarmed strike as you would a weapon.
Are unarmed strikes simple or martial weapons, or perhaps neither?
One thing you should decide is whether you would want unarmed strikes to count as simple or martial weapons, or neither. Fortunately this does not affect proficiencies gained by classes or races as the Player's Handbook Errata states:
However a decision here would impact the spell Tenser's transformation whose description states:
Normally Tenser's transformation would not affect unarmed strikes, but it would if they were classified as either simple or martial.
It would also impact the Swords Bard's Bonus Proficiencies which states:
And so Swords Bards could use unarmed strikes as a spell casting focus (what that means would be up to a GM).
Additionally the Orcish Fury feat states:
And so the feat would now be able to apply to unarmed strikes; however, unarmed strikes do not typically have damage dice as shown in this Q/A so it would only apply if you were a Monk or something else gave your unarmed strikes damage dice such as natural weapons or the Tavern Brawler feat.
And finally the Kensei Monk, which proves to be more complicated. This is because the Kensei Monk's Kensei Weapons feature which states:
This change would allow Kensei Monks to choose unarmed strikes as one of their Kensei Weapons, and from that a few of their features would work differently:
The Kensei Monk’s Deft Strike feature states:
And their Sharpen the Blade feature states:
Both of these features would work with unarmed strikes if the Kensei Monk chose them as one of their Kensei Weapons. In addition, unarmed strikes would now count as monk weapons for the Kensei Monk which would cause two more things to occur:
Their Magical Kensei Weapons feature could apply to unarmed strikes but their unarmed strikes already count as magical from their Ki-Empowered Strikes feature so this wouldn’t create any actual change.
However their Unerring Accuracy feature would now work with unarmed strikes as the feature states:
Remember, the above section of changes would only occur if unarmed strikes were classified as either simple or martial weapons.
Unarmed strikes don’t have weapon damage dice
There are also six features I found which would be affected by this change but for which it is not readily apparent. These are all features which very technically do not work with unarmed strikes because they are not weapons:
A Paladin’s Divine Smite feature states:
As shown in this Q/A Divine Smite does not technically work with unarmed strikes, though they do qualify for melee weapon attacks there is no “weapon’s damage” to be modified as unarmed strikes are not weapons.
Similarly the Savage Attacker feat states:
And so though unarmed strikes qualify as melee weapon attacks, there are no “weapon’s damage dice” to reroll and so they do not work together. This is shown in the Sage Advice Compendium which states:
In much the same way the Half-Orc’s Savage Attacks feature would also not work as it states:
The Barbarian’s Brutal Critical feature states:
The Hunter Ranger’s Horde Breaker feature stats:
And finally the Gloom Stalker Ranger’s Dread Ambusher feature stats:
The above section of features only technically do not work with unarmed strikes as they are not specifically weapons or do not have weapon damage dice, however, making unarmed strikes count as weapons would allow all of these features to use unarmed strikes as well.
Allowing unarmed strikes to count as weapons would provide marginal benefits to a few features by allowing them to be activated even when no other weapons are available or when you would not want to use weapons. This a very small buff to a small subset of features and so it would not be imbalancing.