Ray J Says Fan Stole His Heart Monitor in Crowd During Concert

Ray J's heart monitor has been stolen ... and he's pleading with the thief to give it back.

TMZ caught up with the "Sexy Can I" singer this week -- less than 2 months after he was hospitalized -- and he tells us he's been wearing a heart monitor for the last 2 weeks so docs can keep track of his health. But ... he says a fan snatched the monitor off his bare chest when he hopped into the crowd during his Valentine's Day concert in Shreveport, Louisiana while singing "One Wish."

It's all in the clip -- Ray J says his monitor has important health data his doctors need in order to make a plan for him going forward ... and is even offering cash to the person who has it. He presses that this is no joke and he was wearing it during a "critical" time.

We told you all about it -- Ray J was hospitalized in Vegas in early January and was treated for pneumonia and serious chest pains. He later revealed his docs told him he might not live past 2027.

That's why Ray J's now making sure he's getting his bucket list accomplished ... telling us in our chat this week he's feeling "great" and he's thrilled to launch his comedy career on TMZ's Los Angeles tour bus this Friday.

He gushed about his current and future plans ... even though he admits it might not be what he "should" be doing considering his current health complications.