The 'Glee' star died in 2013 after an accidental overdose.
"Hey you. 10 years. It feels like only yesterday that you were here and yet a million years ago all at the same time," she wrote. "I hold all of our memories in my heart where they will stay safe and never forgotten. We miss you every day and will never forget the light you brought to us all. I miss you big guy."
Michele ended her message with a sweet wish, referencing Foo Fighters drummer Taylor Hawkins, who died in 2022.
"I hope you found Taylor up there and are playing the drums together. 🤍," she wrote.
The Funny Girl actress' message was written next to a throwback photo of her and Monteith cuddled close while she sits on his lap.
Monteith died at the age of 31 in Vancouver, British Columbia. After his death, the BC Coroners Service later determined that Monteith died from mixed drug toxicity, involving heroin and alcohol.
At the time of his death, Monteith and Michele had been dating for over a year.
Last year, during her An Evening With Lea Michele: Life in Music tour, the actress performed Bob Dylan's "Make You Feel My Love," which her character, Rachel Berry, sang during Glee's tribute episode dedicated to Monteith and his character, Finn.
"I don't even remember what I was listening to at the time. Like, nothing. I was just in such a hole... I'm grateful that he [Ryan Murphy] asked me. Because it needed to be something that felt real for me," Michele said, per E!.
Michele told the crowd that shooting the episode had been difficult, but was ultimately helpful in regard to her coping and coming to terms with her loss. However, she admitted that she's never actually watched the finished episode.
"It's the only one I've not seen. Because I think if I don't watch it, it just kind of feels like Finn is still there," Michele explained. "So this [song] is very special."
Michele, who is married to Zandy Reich, honors Monteith each year via social media.
In 2022, Michele shared a throwback picture on her Instagram Story that featured her gazing up at Monteith as they walked side by side. Michele simply captioned the picture with a red heart emoji.