Britney Spears has spent time reuniting with her nearest and dearest in recent years, and now you can add her ex-fiancé, and former co-conservator, Jason Trawick to that list ... TMZ has learned.

Sources with direct knowledge tell TMZ ... Britney got together with Jason during her trip to Las Vegas last week with her brother Bryan. We're told she and her bro are still tight with Jason, and they reached out to him to say they were going to be in town and would love to catch up.

Our sources also tell us Jason now lives in Vegas so the meet-up totally made sense -- and they wound up hanging out together, on at least one occasion, at Resorts World Las Vegas.

Jason was a huge rock in Britney's life during their romantic stint from 2009 to 2013. They got engaged in 2011, and even though they called it off ... we're told they've stayed cordial since the split.

We know what you're thinking, but there's nothing romantic going on between them right now -- we're told it's just friendly.

However, it's also true that some people close to Britney are cautiously optimistic about a possible romantic reunion in the future. To be clear, though, our sources stressed that's not where Brit and Jason's heads are right now.