The 26-year-old was also a contestant on Celebs Go Dating and a former semi-professional footballer for Stevenage.
Fellow Love Island contestants Chris Hughes and Olivia Buckland were the first to pay their tributes on Twitter, saying they were in shock.
Kem Cetinay and Amber Davies, the winners of the 2017, also tweeted their tributes on Twitter, calling it heartbreaking.
His former football club, Stevenage, said everyone at the club was "shocked and saddened".
Love Island host Caroline Flack shared her condolences, saying he was a "total gent".
Thalassitis was born in London and had spells playing football for the Cyprus youth squads at under-19 and under-21 level.
His death comes two months after his best friend Danny Cutts passed away, on Christmas Eve.
It has also been reported that his grandmother, who he lived with in order to care for her, passed away just two days before his death.
Thalassitis was set to open a restaurant called The Skillet in Essex with former professional footballer Scott Neilson, according to filings at Companies House.
In June last year Sophie Gradon from the second series of Love Island passed away.
© Sky News 2019