The description of the monk's Deflect Missiles feature says
you can use your reaction to deflect or catch the missile when you are hit by a ranged weapon attack.
For example, say there's a wall that a goblin is hiding behind. It's my ranged ally's turn and they can see me but can't see the goblin because it's behind the wall. Could my ally shoot an arrow or any weapon projectile at me so I could use Deflect Missiles to deflect it at the enemy?
Best Answer
Yep, that is allowed
Nothing in the rules for the monk's Deflect Missiles feature prevents this sequence of events from working as you describe:
It can be used exactly as you described. Any incoming missile, as long as it is from a ranged weapon attack and hits you, can be deflected. Bonus points for creativity :)
It is kinda expensive though; it expends an attack for your ally, and a reaction and a ki point for you to achieve. It also comes with the risk of simply getting shot if the ranged ally rolls well on the damage roll and you roll badly on the damage reduction from Deflect Missiles.
Just remember that it doesn't work with ranged spell attacks, before you get the party wizard to chuck magic stones at you.