In character as Rachel Berry, Lea addressed her fellow students at McKinley High School as they mourned the death of Finn Hudson (played by Cory).
"I loved Finn and he loved me," she said, struggling to contain her emotions. "He loved all of you guys, I know he did.
"Before Finn I used to sing alone, and this is the first song that we would sing when we would drive around together," she added, before performing a moving rendition of Make You Feel My Love.
Tears flowed down her cheeks as she struggled to finish the song.
In another heartbreaking moment, the actress, who was Cory's girlfriend both on and off-screen, was seen as Rachel saying, "I can hear his voice so clearly. Do you think I'll ever forget it? Because I'm afraid that one day I will."
The whole episode, entitled The Quarterback, was dedicated to Cory, who died on 13 July 2013 as the result of an intravenous heroin combined with alcohol.
Tributes were made all the more poignant given the episode's sorrowful context as the actors said goodbye to both the fictional quarterback, and the young actor who played him.
The hour-long tribute episode aired as Cory's estranged father decided to speak out about being unable to say a proper goodbye to his son.
"It's just ripping my insides out and tearing me apart," Joe Monteith told PEOPLE, as he described how he didn't have much contact with his son while he was growing up.
"One year, she [Cory's mother Ann McGregor] even sent back the Christmas presents that I had sent to the boys," said Joe, who said that Cory told him he "didn't know" that his father had been trying to get in touch.