"The biggest heart and most beautiful smile.. In all of our hearts.. We love you so. Happy Birthday," the actress wrote, sharing a photo of herself laughing along with the actor.
Monteith, who died in July 2013 due to a toxic mix of heroin and alcohol, dated Michele for a few years before his death.
"I only have happy memories of Cory. He was not his addiction — unfortunately, it won. But that wasn't who he was," Michele said in the April 2014 issue of Seventeen.
"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."
The 27-year-old, who plays golden-voiced prima donna Rachel Berry on Fox's musical dramedy, also told Glamour UK that they had plans to grow old together.
"We talked about a lot of things. We talked about children and what we would look like when we grew old and who would be fat and how we would stay thin. We talked about where we wanted to go and what we wanted to do," she said. "We were done. We were it. When you're at that place in your life with someone, you talk about everything. But today I feel like I was given the best part of Cory and I'm thankful for that."
The actress turned to her "Glee" costar
"Now that I had this experience happen to me … we decided to write about it," she said at a listening session in March. "We decided that's what felt organic."
She later went back to the studio to record the album's closer, "If You Say So," which was inspired by the last conversation she had with Monteith. She said the song was uplifting and also what she needed to get her through her "difficult situation."