'90 Day Fiance' Star Ben Rathbun Dead at 55

"90 Day Fiance" star Ben Rathbun is dead ... TMZ has confirmed.

Ben died Monday morning at his home in Greencastle, Indiana, where he was surrounded by his family ... according to a family source.

We're told Ben had stage 4 stomach cancer and he had been battling the disease since the end of last year, when he was diagnosed.

Ben's story was featured on season 5 of "90 Day Fiance: Before the 90 Days" ... and it was an interesting one. At the time, Ben was a 52-year-old father of 4 who met his 22-year-old girlfriend, Mahogany Roca, online. Lots of folks felt Mahogany -- who was from South America -- was catfishing Ben ... but that turned out not to be the case when they met in real life.

Ben and Mahogany appeared to get married after the reality TV show and she was reportedly still by his side at the time of his death.

The Instagram page shabootydotcom was the first to report Ben's death. He was 55.