"Oh, that's very kind, thank you," Trebek said, as the crowd awwed, and sounded choked up as he moved on to the next contestant.
The moment came during the first night of the Tournament of Champions semi-finals, and the contestant who wrote the message of support was Dhruv Gaur.
Trebek has been open about his diagnosis of Stage 4 pancreatic cancer, and the treatment he's receiving for it.
The beloved game show host, who has been helming Jeopardy! since 1984, has received a lot of love on social media ever since he announced his diagnosis.
The clip of Gaur's heartfelt 'guess' inspired a Twitter trend in the United States of #WeLoveYouAlex, with many sharing the video and saying it made them cry.
Guar later tweeted about why he decided to answer with a message of love, saying that Trebek had just told the contestants he was reentering cancer treatment. "We were all hurting for him so badly," he wrote. "I just kept thinking about Alex, and thought he should know."