Canseco, 54, accused Rodriguez of cheating on Lopez with Canseco's ex-wife, Jessica Canseco, to whom he was married from 1996 to 1999.
“Watching World of Dance watching J.Lo text Alex Rodriguez little does she know that he is cheating on her with my ex-wife Jessica poor girl she has no idea who he really is,” Canseco tweeted.
He then claimed he was with his ex-wife a few months ago when Rodriguez called her on the phone.
“Alex Rodriguez stop being a piece of s--t stop cheating on Jennifer Lopez,” Canseco tweeted before challenging Rodriguez to an MMA fight.
Finally, Canseco offered to take a polygraph test to prove that he’s not lying about the allegations.
Canseco famously mentioned Rodriguez in his book “Vindicated” – a sequel to his jaw-dropping book “Juiced – claiming he introduced Rodriguez to a steroids distributor and that Rodriguez slept with his wife at the time.
Rodriguez didn’t immediately respond to a request for comment by Fox News.
The remarks came a day after Rodriguez and Lopez announced their engagement. The pair celebrated their two-year anniversary Feb. 4.