Museum staff believe Gus, who is barely bigger than a kitten, is the oldest gopher turtle in the world.
They estimate he hatched sometime between 1920 and 1925.
His guests were treated to birthday cake and lemonade, but Gus himself dined on organic blueberries and strawberries.
The fruit was a "special treat" for the almost-century-old tortoise.
Jeff Gray, the museum's curator of marketing and communications, said Gus usually sticks to a balanced diet of lettuce, berries and bananas.
Gus spent a large part of his birthday in his habitat at the museum, but he came out for a round of birthday wishes.
Children sang happy birthday and drew hearts and well wishes on his birthday card, but the real celebration came when Gus went for an afternoon walk.
It Gus's opportunity to interact with his guests, something Gray said the tortoise loves to do.
"He really does love people... he's a little slow in the morning getting up but once he gets going by mid-afternoon he seems to really interact well with visitors," Gray said.
Gus didn't seem too motivated to walk around, preferring instead to sit and watch his birthday festivities.
But he was treated to lots of inquisitive pats from his younger guests.