Katy Perry says Jelly Roll would make a good 'American Idol' replacement before she dips -- this as the question of who's gonna fill her shoes continues to linger.

The singer told E! News she's all in for the rapper/country star to take her spot next season -- especially since he made a lasting impression on her during his guest appearance on the ABC show earlier this month.

She says "I gotta say, Jelly Roll was crazy when he came on the show" ... adding he had her totally convinced with all his industry knowledge.

In fact, Katy was so impressed with JR she quipped he could run for president and even become her pastor -- and she'd totally buy it!

But, when it comes down to it, Katy’s looking for a “truth teller” to take her spot -- and someone who doesn’t get tangled up in online comments, a game she knows all too well.

Katy explained being a female with an opinion and being the boss sometimes came with the "B-I-T-C-H" label ... so her successor needs a strong sense of self, which she believes Jelly Roll has. He recently appeared on an 'Idol' episode, and clearly left an impression.

Confidence isn't something JR lacks -- dude speaks his mind and then some, on several different topics, in fact.. Anyway, unclear if Katy has much of a say on this (probably not, TBH) but she's at least weighed in with a name now.

We'll see who ABC ends up picking, and whether Lionel Richie and Luke Bryan's opinions will be considered.