WHO KNEW? MICHAEL PHELPS IS A JAY Z FAN!

Michael Phelps is turning over a new leaf: The former hard-partying Olympian was on his best behavior at Jay-Z's concert in Atlantic City on Saturday night.

Phelps, who has gotten heat over the past year for gallivanting at Marquee club in New York and the Palms in Las Vegas, seemed especially low key as he partied with a group of guy friends at the Borgata's intimate show.

"He stayed very under the radar throughout the concert and even later at the club," one guest noted of the concert's after party at club mur.mur.

"He kept a hat on and was only drinking casually. He was really nice to girls that wanted to take pictures with him, but would excuse himself right after. He looked like he didn't want any attention."

So has the 24-year-old learned his lesson?

An insider adds that Phelps has tried to change his ways since his partying (and pictures of him doing so) got him in trouble with his sponsors, coaches and his fans — namely a photo that surfaced in early 2009 of him taking a bong hit in November of 2008.

"He's really trying to put that behind him," says the source.

Other very-famous faces looking like they were having just a little bit more fun at the event included Stevie Wonder, Robin Thicke and Jay's lady love Beyoncé — who all joined Hova and partied throughout the evening.

After the concert, Jay-Z, Wonder and Thicke had a boys' dinner at Old Homestead, where they caught the Pacquiao-Clottey fight, and the trio were later seen having cocktails with deejay (and notorious Lindsay Lohan ex) Samantha Ronson.

But by all accounts Jay looked the most thrilled, and "Crazy In Love," when he spotted Beyoncé arriving to the venue around midnight.