"I think for me it's better if I stay retired," GSP tells TMZ Sports ... "It's not worth it for me."

Of course, GSP formally retired back in 2019 -- saying he wanted to leave while still on top (his last fight was a '17 victory over Michael Bisping).

But, 39-year-old GSP still trains like a maniac -- and people close to him say he's still in dangerous fighting shape.

In fact, UFC welterweight champ Kamaru Usman called him out earlier this month -- saying he would love to bait GSP out of retirement for one last fight before he truly rides off into the sunset.

But, St-Pierre tells us he's given it some serious thought ... and he just isn't feeling a comeback -- at least not right now.

"Sometimes I'm watching the fights and I'm like, 'Man I think I can beat these guys' or after when I'm done training I feel like I'm very confident," GSP says.

"But, after I go home at night and I'm thinking about all that crazy thoughts and I'm like 'I'm gonna be 40 years old soon, do I really wanna go back and do a hardcore training camp of 2 months? Put myself in jail so to speak and get ready for hell? I'm like, no I don't."

"I've done it for a very long time and unfortunately there's too many guys that retire too late and I do not want to be one of these guys."

"I'm very fortunate, I'm healthy most importantly and I'm also wealthy. I came out at the right time, I made a lot of money and I think the problems are when I matured and calculated the pros and cons, it's better that i stay retired."

GSP had nothing but praise for Usman -- saying the guy is one of the smartest fighters out there and has big things in his future ... but he's just not ready to throw down right now.

But, watch the whole clip ... St-Pierre says he's always willing to entertain an offer if the UFC calls him up.