Brault stood behind home plate to sing "The Star-Spangled Banner." His Pirates teammates came onto the field to watch him and congratulated him when he was finished.
The left-hander had previously sung the national anthem in the minor leagues, but Tuesday's performance was his first as a major league pitcher.
"It's something specifically my grandma always wanted me to do," Brault said on Monday, according to MLB.com. "She wanted me to sing the national anthem before a major league game that I got to play in, so that's part of it."
He said Monday the other reason he wanted to sing the national anthem was "showing people, showing kids, that it's OK to do other things. I think that's really important. It's something I really live by."
Brault, 26, is in his third season with the Pirates. He is 5-1 with a 4.38 ERA in 51? innings over 19 appearances, including five starts.
He did not pitch in Tuesday's game, as the Pirates fell to the Brewers 3-2.