The powerful tornado packed winds of more than 200 miles an hour, destroyed some 7500 buildings and killed 161 people. It was one of the deadliest tornadoes in U.S. history.
Mr. Obama spoke to high school seniors at their graduation ceremony Monday.
"You're from Joplin, so you'll always know that it's always possible for a community to come together when it matters most," he said.
Last May's tornado destroyed the high school and forced seniors to spend their final year in a converted store at the city's shopping mall.
A four-mile unity walk through some of the city's hardest hit neighborhoods was planned for the afternoon.