Patrick Mendes claims his estranged wife, Thais Ramone, stepped out on him ... and when he discovered her alleged affair, he claims she kicked him out of the house.

The "90 Day Fiancé" star made the allegations against Thais in a new legal document filed Wednesday ... telling the court he found out about Thais' alleged affair on or around April 11, 2026.

He says that after he found out Thais was cheating on him, she demanded he leave their marital home. Patrick says he vacated the house on May 6.

A couple of weeks after that, Patrick says Thais threatened him with a knife ... while holding their three-year-old daughter, Aleesi. Patrick says he called the cops and Thais was arrested.

Patrick previously asked for sole legal and physical custody of Aleesi, but it looks like he's changed his tune ... because in this doc he's asking for joint legal and physical custody.

We reported on Thais' arrest earlier this year ... and we showed you the police body cam footage.

According to the police report, Patrick accused Thais of slapping him -- but apparently made no mention of the alleged knife threat.

We reached out to Thais ... so far, no word back.