It's all spelled out in court docs, obtained by TMZ Sports, in which Steph Curry's father says Sonya is, and has been, dating Steven Johnson -- a 6th-round pick in the 1988 NFL Draft.

In the docs, Dell says Sonya "began her extramarital affair with Mr. Johnson during the marriage and prior to the date of separation, and she lied to [Dell] each time she cheated on him."

Dell, in the docs, says Sonya is actually living with Johnson in Tennessee ... and claims she should not be entitled to alimony because of it.

For her part, Sonya said in responding documents that she is NOT living with Johnson -- claiming she's currently living on her own because Dell will not allow her to live at their home.

Sonya added that she is, however, currently in a dating relationship ... but denied cheating on Dell, saying the relationship began "months after" she and Dell agreed to legally separate in March 2020.

Sonya also said Dell cheated on her during the marriage ... claiming he hooked up with different women, and that family and close friends knew about it.

As we previously reported, Sonya -- who married Dell in 1988 -- filed for divorce back on June 14.

Johnson, meanwhile, was a solid player for Virginia Tech from 1984 to 1987, playing in 44 games while piling up 1,058 receiving yards and 8 receiving TDs.

The 56-year-old went on to play just one year for the Pats, starting 3 games in the 1988 season.

According to Johnson's business page, he's a fan of "road racing, fishing, golf, and travel."