Monteith would have turned 32 on Sunday.
"The biggest heart and most beautiful smile," Michele wrote. "In all of our hearts.. We love you so. Happy Birthday."
She also shared a sweet photo of herself laughing with the late Glee star.
Michele told Seventeen magazine last month that Monteith was "not his addiction."
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
"I only have happy memories of Cory," she said.
"He was not his addiction -- unfortunately, it won. But that wasn't who he was. 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."