He also tossed the offering sans-beard.
Wilson, 36, played for the Giants from 2006 through 2012, before joining the Los Angeles Dodgers in 2013. The three-time All-Star led Major League Baseball with 48 saves in 2010. Wilson played his final season in 2014 for the Dodgers. Wilson won a World Series in 2010 with the Giants.
He was on hand Tuesday before the Giants hosted the Seattle Mariners at AT&T Park in San Francisco. Wilson tossed the pitch into former teammate and Giants All-Star catcher Buster Posey. He sported the same uniform that the Giants were wearing for their first home contest of 2018.
Wilson walked out for the sequence to his token House of Pain music, which he used during his playing days. The only thing missing from the moment was Wilson's significant beard.
"Well this looks familiar. World Champion #SFGiants closer Brian Wilson is in the house. #SFGOpeningDay," the Giants tweeted.
The Mariners led the Giants 6-3 after seven innings.
"[I miss] the smell of the ballpark," Wilson told reporters. "I didn't get any sleep last week. Major jitters. I have to throw the first pitch and I got two days warning. What? Ok, yeah I'll do it, but I don't know."
Wilson said he just felt like wearing his uniform for the pitch, including the cleats he was fined for during his playing days.