John Glenn tossed out the ceremonial first pitch before the Cleveland Indians played the New York Yankees on Sunday.
The 91-year-old Glenn was the first American to orbit the Earth in 1962.
He asked the crowd for a moment of silence to honor Neil Armstrong, the first man to walk on the moon, who died Saturday at age 82.
The Indians scheduled the appearance months ago to celebrate the 50th anniversary of Glenn's historic space flight on Feb. 20, 1962.
Glenn called Armstrong a true patriot and spoke fondly of Hall of Famer Ted Williams. Glenn and Williams flew missions together during the Korean War.
Glenn later became a US senator from his native Ohio.