A monk's Deflect Missiles ability (PHB p. 78) triggers on the following circumstance:
Starting at 3rd level, you can use your reaction to deflect or catch the missile when you are hit by a ranged weapon attack.
Does this work against thrown melee weapons, such as a javelin or dagger?
Best Answer
Yes
The Sage Advice Compendium has clarified what is meant by "ranged weapon attack" (italics added).
If you throw a melee weapon, you are making a ranged attack, and you are using a weapon. Thus, you are making a "ranged weapon attack." Therefore, the attack is a valid one for a monk to attempt to "deflect".
Note that this definition applies even if the weapon in question doesn't have the "thrown" property (although it would then be counted as an "improvised weapon", it's still a weapon that is being used to make a ranged attack.)