The footballer turned reality star is reported to have been found dead in woodland near his home today (Friday, March 15).
The cause of death is not yet known.
Mike rose to fame on 2017's Love Island, where he landed the nickname 'Muggy Mike', before going on to make magazine headlines for his relationship with The Only Way Is Essex's Megan McKenna.
He also appeared in E4 reality dating series Celebs Go Dating last year.
Mike's fans and his fellow Love Island stars have quickly taken to Twitter to pay tribute.
Chris Hughes, who appeared on Love Island with Mike, wrote: "His name wasn’t Muggy Mike. Please stop brandishing that. His name was Mike Thalassitis, and every inch of my heart goes out to his family & friends. A good guy, taken far, far to soon. 💔'
Tyla Carr, who coupled up with Mike while on Love Island, said: "I can't believe the call I've just had. Mike, you were loved by all of the Love island family. You'll be missed so much. All my love goes out to your friends and family. Shocked."
2018 Love Islander Dr Alex George shared: "Devastated to hear of the passing of Mike Thalassitis. My heart goes out to friends and family ❤️"
Kady McDermott, from Love Island series 2, posted: "In absolute shock. Mike Thalassitis your lovely self and smile will be truly missed. I send my love and condolences to all of his friends and family at this awful time. Will never be forgotten. RIP 💔"
Her fellow Love Island contestant Olivia Buckland posted online: "This is horrendous. I’m in shock. Mike you are going to be sorely sorely missed. My heart is with your family. I’m in complete shock ❤️"
And Love Island host Caroline Flack took to Instagram to say: "Mike... you were a total gentleman... I won’t forget that ... it’s so so sad ...you will be missed. RIP. ❤️"
A spokesman for ITV said: "Everyone at ITV2 and Love Island are shocked and saddened by this terrible news.
"Our thoughts and deepest condolences are with Mike's family and friends at this very sad time."
In a statement, one of Mike's former football clubs Chelmsford City FC also posted: "The Club are saddened to hear the news of the passing of former #Clarets striker Mike Thalassitis. Our thoughts and prayers go to his family and friends at this time. #ClaretTogether"
Stevenage FC, where Mike also played, added: "Everybody at Stevenage FC is shocked & saddened to hear the tragic news about former player Michael Thalassitis. Our sincerest condolences go to his family & friends."
If you ever need help, Samaritans offer confidential advice and support, 24 hours a day, 7 days a week over the phone on 116 123 or online