Bieber wore a shirt carrying a photograph of the actress in character as Kelly Kapowski in Saved By The Bell to the MuchMusic Video Awards in June.
Thiessen said on today's Fox and Friends: "When I got the call he was wearing my T-shirt it was hilarious because I don't think he was even born when the show was on, it had gone off the air.
"It kind of made me feel even older like, geez, now I'm ancient! It's a vintage T-shirt!"
Saved By The Bell, which spawned a series of spinoff projects, aired between 1989 and 1993. Bieber was born in 1994.
Thiessen later returned the favour by wearing a T-shirt featuring an image of Bieber to the premiere of Horrible Bosses in New York City, telling Vulture: "I'm just trying to show the love back, that's all."
Thiessen, who welcomed daughter Harper Renn last year, can be seen in the third season of White Collar, which airs Tuesdays at 9/8c on USA.
"It's been great," she added to Fox and Friends. "It's been a very busy last year. The show is in its third season. We're shooting right now and it's going great. It's been fun."
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Watch more of Tiffani Thiessen's interview on Fox and Friends below:
Watch a trailer for the third season of White Collar below: