is celebrating a milestone birthday -- and looking fabulous doing it!

On Tuesday, the actress took to Instagram in honor of her 50th birthday, sharing a sexy photo of herself posing on a bed with only a white bed sheet covering her chest and torso.

"Over the hill. #thisis50," Thiessen captioned the shot.

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Many of the alum's celebrity pals flocked to the comments section of the post to share their birthday wishes.

Fellow alum Mario Lopez wrote, "Welcome to the 5th level kid!"

"Happy Birthday! I remember knowing you as a teenager and hoping to be as gorgeous as you one day!" Jennifer Love Hewitt added. "And I still feel the same! You make 50 look great! ❤️."

"Well done," Jenna Fischer commented. "Starting the 50s exactly right. Happy Birthday!!!,

"Looks pretty 🔥🔥🔥 to me," Sarah Michelle Gellar wrote, while Tamera Mowry said, "You look amazing! 🔥🔥🔥."

Valerie Bertinelli also chimed in, calling Thiessen a "hot mama."

Thiessen's husband, Brady Smith, reacted to his wife's post as well, writing, "Holy guacamole babahhhhhhhh!!!! Hubba Hubba."

Thiessen shares daughter Harper and son Holt with Smith, whom she married in 2005.

Meanwhile, during an appearance on in October, Thiessen shared her thoughts about turning 50.

"I can't believe it. I actually really cannot believe it. I cannot believe I’m going to be turning 50 very soon," she said. "I feel great. I actually feel more settled than I did in my 40s. Turning 40 was a bigger deal. I’m like, 'Ooh, I’m going to be 50!' I still can't believe it."

The star noted that she's "happy" with all parts of herself, even her wrinkles, noting that she's "earned" them.

"I say earned because I think I've earned them," Thiessen said. "They're from all the tears and laughter -- both! And I’m happy about that.

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