A$AP Rocky says his time behind bars in Sweden is exactly why he's returning to that country -- which seems insane -- but he's on a mission to help the inmates and immigrants he met there.

We got the rapper Tuesday evening outside the swanky Rick Owens clothing store in NYC and asked why in the world he'd go back. A$AP says the month in jail for his assault case opened his eyes to the plight of immigrants. He says he was especially aware of the support he was getting, as a celeb, while some prisoners around him were rotting for 10 months or more before seeing a judge.

He's on a mission to change that. For starters, A$AP says he's donating proceeds from his December 11 concert in Stockholm to an immigrant rights org called FARR, the Swedish Network of Refugee Support Groups.

As we first reported ... Rocky was held in inhumane conditions while prosecutors decided whether to charge him for aggravated assault stemming from a street brawl. The case drew worldwide attention ... and even President Trump and his administration got involved.

A$AP was ultimately found guilty but did NOT have to serve any more time behind bars. Rocky wasted no time jetting outta Sweden, and it seemed that would be the last time he'd ever be there -- until Live Nation announced he'd be returning to Sweden next month.

A$AP had said jail was scary and humbling during his first performance in the States after getting back from Sweden. Still, he tells us he's not letting fear hold him back, and even plans to visit an area where police don't go ... just to boost spirits of Swedish immigrants.

There was one topic he did NOT want touch, though. We asked about Trump's efforts to free him, and ... well, just watch. Fair to say, Rocky would rather play a Swedish jailhouse concert than discuss POTUS!