The 31-year-old actor reportedly died due to a mixed drug toxicity of heroin and alcohol.
The Canadian actor, who played school jock Finn Hudson in the hit U.S. musical series, was found dead in a Vancouver hotel room in 2013, aged 31.
"Hard to believe it's been 4 years ..."
Fans rallied around the star as she remembered her late boyfriend on Thursday, offering their well wishes.
"We miss you C..."
The actor previously revealed that he had "a serious drug problem" when he was younger.
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Taking to Twitter, Michele shared a photo of the former couple with a simple, heartfelt caption.
The actress has since gone on to date other actors, Matthew Paetz and Robert Buckley and is now starring in upcoming series The Mayor.
"I love working with Cory, I always have", Micheletold PEOPLE in September 2012.
At the time, the actress confessed that dealing with his death was not an easy task and that she continued to struggle.
Michele and Monteith were still dating at the time of Corey's sudden death on July 13, 2013.