Leonardo DiCaprio regrets not hooking up with Rollergirl and Amber Waves back in the 1990s … admitting he wishes he took the lead role in “Boogie Nights.”

The actor sat down with Paul Thomas Anderson — director of the iconic 1997 film about the porn industry, who also directed Leo in the upcoming flick “One Battle After Another” — for an Esquire interview … and, he says he regrets turning down the lead role in a “profound movie of my generation.”

DiCaprio admits he can’t see anyone besides his pal Mark Wahlberg — who he actually recommended for the role after they appeared in “The Basketball Diaries” together — in the part, and he calls the movie a masterpiece … but, he would’ve liked to be in it.

If you’re wondering why he turned it down, Leo was busy with a different role … a little film you might know called “Titanic” — which is a pretty iconic movie from DiCaprio’s generation too.

DiCaprio and Anderson talked about why it took so long for the two of them to work together … and LD says he’s been trying to work with PTA for two decades — and “One Battle After Another” was just the right film to make together.