"The biggest heart and most beautiful smile.. In all of our hearts.. We love you so.. Happy Birthday," she posted, sharing a black-and-white photo of the couple laughing together.
Monteith would have turned 32 on Sunday. The Glee actor was found dead in a Vancouver hotel room in July after overdosing on heroin and alcohol, and the loss of such a young talent sent shockwaves through Hollywood.
Michele, 27, was especially heartbroken: The two dated on- and off-screen, and she has called him her "true love."
"He was not his addiction – unfortunately, it won. But that wasn't who he was," she told Seventeen's April issue. "Cory made me feel like a queen every day. From the minute he said 'I'm your boyfriend,' I loved every day, and I thank him for being the best boyfriend and making me feel so beautiful."
Monteith's mom, Ann McGregor, also Tweeted a sweet message in memory of her son.
"maybe it is a blessing that Cory's birthday is on Mother's Day," she wrote Friday.
The biggest heart and most beautiful smile.. In all of our hearts.. We love you so. Happy Birthday. pic.twitter.com/XzGVu0g5nY— Lea Michele (@msleamichele) May 11, 2014