R. Kelly's fiancée, and the woman the world believed was his victim, Joycelyn Savage, just dropped a bomb ... claiming to be pregnant with the singer's baby.

Joycelyn reveals in her upcoming book she's several months pregnant, learning she was expecting after the disgraced artist sent her an engagement ring. She says Kelly is "extremely excited about the news," but wishes he could be there for the birth of their kid.

The big hole, obviously, is she never explains how the child was conceived -- R. Kelly has been locked up for over a year.

A lawyer for Joycelyn's parents tells TMZ they're still processing the news, but aren't willing to make a comment at this time.

As we reported, Joycelyn revealed the two were engaged through a letter sent to the judge ahead of his sentencing, asking for him to get a lenient sentence while claiming, "I'm not the victim that the government has portrayed me to be."

In her book, she refers to Azriel Clary, one of Kelly's other live-in GF's as "the other girl" ... saying she became bipolar and eventually "smeared my name all over social media" after returning to her family.

As you know, Azriel wanted out of the situation and tried getting Joycelyn to leave, too ... resulting in them landing blows on each other back in 2020.