Tom Brady, Sofia Vergara Both Single, Hanging Out, but Not Dating

Tom Brady and Sofia Vergara are having a summer fling ... but they aren't dating, and it's not a romance ... TMZ has learned.

Our sources say TB12 and the actress are both single and are definitely not a couple, but they are having fun on a trip through Europe with a bunch of other celeb travelers.

Tom and Sofia's summer fling is just getting started ... about a week ago, they hung out on a superyacht with Martha Stewart, Kendall Jenner, Naomi Campbell, Kate Hudson, Patrick Schwarzenegger, Anitta, Tessa Thompson and one of Tom's former flings, Irina Shayk. The two-night excursion launched from Rome, Italy.

TB12 moved his seat on the yacht to sit next to Sofia during a dinner ... and Sofia posted a photo of them seated at a table together, with J Balvin coming by to say hello and give her a smooch on the forehead.

There are reports describing Tom and Sofia's situation as a "summer romance" .... but sources familiar with the situation say to call it a "romance" is "ridiculous" and, at least for now, it's nothing more than a summer fling.

How long this thing lasts, however, is anyone's guess ... and Tom hasn't had a steady, public relationship since his divorce from Gisele Bundchen, who has moved on with Joaquim Valente.

Tom's been in Europe for a minute now ... and he was one of the bigger party animals at the Jeff Bezos wedding in Venice ... though we were told it was more of a bro-fest than a hook up scene.

Sofia's also made a stop in Ibiza ... and we saw Tom there on a yacht with his 12-year-old daughter.