The singer-actress, popularly known as JLo, became the latest in a long line of people to imitate Monroe when she famously sang Happy Birthday to John F Kennedy in 1962, Daily Mail online reported.
The 40-year-old singer made a guest appearance on Lopez Tonight to promote her new film The Back-up Plan and oozed old Hollywood glamour as she sang for George Lopez.
As George, who turned 49 on Friday, was wrapping up the show, the cameras faded to black and the audience heard J-Lo's voice coming from backstage.
"I wanted to do something special to surprise you. Take a seat and let me do my thing," said J Lo.
In a blonde wig and tight-fitting silver floor-length gown, she then tottered on stage and took her place behind an old radio hall microphone before breathlessly singing 'Happy Birthday To You' to the lucky host.
As the audience clapped loudly, a giant birthday cake was wheeled onto the stage as hundreds of balloons fell from the ceiling.
Photograph: Mario Anzuoni/ Reuters