Koepka won his first four majors in the space of eight events from 2017 to 2019, but suffered career-threatening injuries and a loss of form before joining LIV Golf last year.
The former world number one also felt, in his own words, that he had "choked" by failing to convert a four-shot lead after six holes of the third round of the Masters at Augusta National.
There was no repeat of such failings at Oak Hill as Koepka held off the challenge of Viktor Hovland and Scottie Scheffler to claim a third US PGA title and move into the automatic qualifying places for the US Ryder Cup team.
"It feels damned good," Koepka said. "Yeah, this one is definitely special.
"I think this one is probably the most meaningful of them all with everything that’s gone on, all the crazy stuff over the last few years. But it feels good to be back and to get number five.
"When I look back at where we were two years ago I’m just so happy right now. I’m kind of at a loss for words. This is the coolest thing.
"There’s a lot of blood, sweat and tears that have gone into this one. Life has changed a lot for me but this one is super gratifying for me.
"I don’t know how many guys have won five times but to be in with those names is incredible. I’m not sure as a kid I even dreamed about doing it."