Colbert, 51, traveled back home to James Island, South Carolina, to meet up with his fourth grade teacher Ms. Katsos (or as she's known now, Mrs. Keelin), whom he admitted to once having a crush on.
She remembered him as "one of the brightest students" she ever taught, calling him an "avid reader" with a big vocabulary.
Colbert asked if she saw any of that little boy still in him.
"There's still a lot of boy in you Stephen in that you really love life and you enjoy people. And you were dramatic, a little bit," she responded. "So the drama still comes out."
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Colbert told her he remembers wanting to make the teacher laugh during a test after school in 1974, and Keelin surprised the TV star by bringing out a cassette tape she had recorded of the test.
Colbert joked that she was the last teacher he ever tried for and the rest of his academic career didn't turn out well.
But, "everything came out wonderful," said Keelin of her famous former student.
Colbert reunited with Keelin in honor of his work with DonorsChoose.org, which connects teachers in needy communities with donors to help.
In his Time cover story in August, Colbert spoke of looking forward to getting to meet up with his former teacher.
"I had such a crush on her. I'm going to talk to her. I haven't seen her since 1974, but I can't believe that they found her," he said. "She moved away when I was 10 and then she came back just recently, so they found her down in Charleston."
The Late Show with Stephen Colbert airs weekdays (11:35 p.m. ET) on CBS.