The supermodel attended her ex-hubby's concert at MSG Friday -- where she was filmed dancing along to Billy's hit "Uptown Girl" ... which was written about Christie back in the day.

Check it out ... Christie is seen grinning widely and swaying back and forth on the Jumbotron while watching her ex sing out his hit song. On its face, she was all about it.

Fans couldn't get enough of the exchange ... with one viral response on TikTok noting, "40 years later and your ex man is still grinning at you like that while he sings the song he wrote about how rich and hot you are? Voodoo clam."

For those unfamiliar ... Christie famously starred in Billy's music video for the upbeat anthem. Although, if you ask Billy's other ex, supermodel Elle Macpherson ... she was the real muse for the single -- but that's another story for another day.

Christie and Billy were married in 1985 ... welcoming daughter Alexa Ray Joel later that same year. However, they eventually called it quits in 1994 -- remaining friends all these years later.

It's a good thing Christie caught Billy's show when she did ... as the crooner is set to close his NYC residency this summer. Remember, back in November, Billy confirmed his final show at the famed arena would be July 25 ... which marks his 150th-lifetime show at the venue.