"Surprise! Welcome to my house," said Mrs. Obama with dog Bo in tow.
PHOTOS: 25 Reasons Michelle Obama Rocks
The surprise was broadcast live on the White House's official website.
After greeting guests with hugs, Mrs. Obama mingled with the crowd of 100, even telling one visitor, "I've never seen sparkly Uggs before." She also talked about healthy living and her "Let's Move Campaign." "Tell your grandbabies I said to eat your vegetables," she joked.
PHOTOS: Michelle Obama "Let's Move Campaign"
One young tourist mentioned it was her birthday and Mrs. Obama said, "That's why I came down. Have a very cool 13th birthday."