Cute crooner John Legend cheated on his Sports Illustrated swimsuit model fiancée, Chrissy Teigen, by smooching a young woman the other night at a Manhattan hotspot, Page Six has exclusively learned.

Legend’s lusty lip-lock came days before Teigen complained via Twitter about other women who want to “[bleep] my fiancé.” And Legend has this week been talking about his impending marriage, and baby plans with Teigen in interviews.

Sources tell us the kiss came when Legend was at Acme on Friday night. A spy said the suave performer was partying with male pals who struck up a conversation with “two pretty girls in their late 20s, one blond and one brunette” at the next table. After “chatting all night,” the witness said the blonde got up to go to the bathroom after 1 a.m., and “John followed her.”

Legend was then seen pulling the woman into a bathroom with him. “He was walking in, and just grabbed her arm and brought her with him,” said the witness. Another witness also confirmed seeing the “Green Light” singer pull the lady into the bathroom, and later overheard the bathroom babe telling her friend that she and Legend shared “a steamy make-out session.”

“They were in there a couple minutes,” said a source. But, the union was apparently short-lived.“The same girl saw another hot guy and started talking to him,” one spy said. Reps for Legend and Teigen had no comment.

Teigen, the host of VH1’s “Model Employee” who recently got into hot water with Instagram for posting a nude pic of herself, tweeted Monday: “I honestly think so many more of us would get along if you didn’t want to [bleep] my fiancé.” Legend, who’s been engaged to Teigen since 2011, said in an interview this week, “There’s a song on my album that actually could be pretty good for the wedding. ‘All of Me’ . . . it’s inspired by Chrissy.”

The two attended a screening together Tuesday night, and, “They seemed very much together,” a spy said. “They were beaming and happy and holding hands.”