Monteith was 31 when he died in his room at the Fairmont Pacific Rim hotel on July 13, 2013.
A coroner ruled he died of mixed drug toxicity involving “intravenous heroin use combined with the ingestion of alcohol.”
The actor, who was born in Calgary and raised in the Vancouver area, struggled with addiction for years and had recently completed a stint in rehab when he died.
Monteith’s estranged father Joe, who lives in New Brunswick, told ET Canada: “I still miss him very much. I still have the empty feeling inside.”
READ MORE: ET Canada looks back at Monteith’s death
Monteith’s girlfriend, Glee star Lea Michele, marked the sad anniversary by tweeting a snapshot of Monteith with the message: “We hold you in our hearts today, and every day we remember your smile. We will love you and miss you always.”
Last December, Michele remembered Monteith during an appearance on Ellen.
“I’m trying to do my best for him because I know that’s what he would have wanted,” she said.
“I literally lived every day of my life feeling like the luckiest girl. I just thought he was the greatest man.”
Among those who retweeted Michele’s message on Sunday was Glee star Chris Colfer.
Here’s a look at how show creator Ryan Murphy and some of Monteith’s former co-stars remembered the actor on Sunday.