"You just never know who you are going to inspire, you just never know," she told them. "You are certainly inspiring me every day. I still have those same feelings of pride, excitement and wonder, so being here is otherworldly for me."
She told the Olympians, "Try to have fun. Try to breathe a little bit. But also win, right? In the end, winning is good."
Obama, who is leading the presidential delegation to the Olympic opening ceremony, arrived in London on Thursday night.
Also on Friday, Obama co-hosted an event with Nickelodeon for her "Let's Move!" anti-obesity initiative with about 1,000 American military children along with American and British students at the U.S. ambassador's residence. Olympians and athletes including soccer star David Beckham, along with a SpongeBob SquarePants representative, joined the first lady for the event.
"Let's Move!" live-tweeted the event from the organization's official Twitter account, sharing photos — including one of Obama on the soccer field with SpongeBob playing goalkeeper — and updates on the first lady's game of tennis with gymnast Shawn Johnson and NBA drills with the Los Angeles Clippers' Grant Hill.
The first lady herself tweeted after the event, saying she had "so much fun" and signing the tweet "-mo."
Obama will later attend the Olympics opening ceremonies, which will be televised in the United States on Friday night.
Updated at 10:02 a.m.