The spell requires 1 Reaction to cast on a creature that has damaged me.
In this scenario, let's say I attack a Warlock. He then casts Hellish Rebuke on me as his reaction. In response to his Hellish Rebuke, can I cast Hellish Rebuke on him too, even if it's technically still in my turn? (I don't know whether Reaction can only be used on other player's turn or not)
Best Answer
Yes - you can use a reaction on your turn to cast this if you have one available
Since hellish rebuke takes a reaction to cast, it obviously qualifies.
The spell specifies that it can be cast
A common cause of getting damaged on your own turn is getting hit by an opportunity attack as a result of your movement1. This would be a valid trigger for the casting of hellish rebuke.
So, if it is your turn and you get damaged and you have a reaction you can cast hellish rebuke as a reaction.
Note: No bonus action spells on your turn before or after hellish rebuke
Note that this means that you cannot cast a bonus action spell on your turn after casting hellish rebuke. If you have cast a bonus action spell already, you cannot cast hellish rebuke during your turn at all2. However, casting a bonus action spell on your turn would not affect your ablilty to cast hellish rebuke as a reaction outside of your turn. This is confirmed by Jeremy Crawford here.
Note: This means you spend your reaction
If you use your reaction on your turn, you are not able to use another one until the start of your next turn3.
1thanks @NautArch
2thanks @goodguy5
3thanks @SteveBauer