Joey Lawrence carried on an extramarital affair with an actress he starred alongside in an upcoming movie ... at least according to her estranged husband ... and it may provide a clue for why Joey's wife just filed for divorce.

Actress Melina Alves' estranged husband, Edward Rider, makes the adultery accusation in new documents from their divorce in New Jersey ... and the timeline here is revealing.

TMZ broke the story ... Joey's wife of 2 years, Samantha Lawrence, just filed to divorce him, and she lists June as their date of separation.

Now, get this ... in new docs obtained by TMZ, Melina's estranged husband claims he found out in late March that Joey and Melina were banging. He makes the allegation in divorce docs filed in late July, a few weeks before Joey's wife filed to end their marriage.

What's more, Joey and Melina are accused of having the affair in California ... which is pretty interesting, considering they star alongside each other in the upcoming movie, "Socked In For Christmas," which was presumably filmed in Hollywood.

Joey's got a starring role in the flick, while Melina's the writer on the project and has an acting credit as well.

And apparently Melina's estranged husband doesn't think the alleged affair was short and sweet either ... alleging the affair went on "at various other times and places."

Melina even featured Joey on her Instagram page, but she recently deleted a photo of them together.

Joey's got a history of getting romantic with women on movie sets ... he actually met his current estranged wife while filming a Lifetime movie together. Both he and Melina have kids with their now-estranged spouses.