Just hours before Jose Canseco allegedly committed rape ... the alleged victim posed for a very flirty pic with J.C. in a Vegas casino and joked that they had just gotten married, TMZ has learned. 

The pic was taken on the floor of the MGM Casino at 2:51 AM on May 10 -- with the woman cozying up for a couply shot with Jose ... with her open right hand placed on the center of his exposed chest.

The person who took the photo says he's a huge Canseco fan (apparently those still exist) ... and had recognized Jose strolling around the casino with a female companion.

The fan asked Jose if he could snap a shot -- and Jose obliged, striking a pose with the woman while she giggled and told the photog she and Jose had just tied the knot.

For the record -- we've done some digging, but couldn't find any proof of a Canseco marriage.

The photog told us the two seemed happy ... no signs of distress.

Of course, things changed -- 'cause the woman reportedly told police Jose drugged and raped her later that day.

Jose has denied the allegations -- claiming his accuser is making up the story for publicity.