Harrison posted a photo on Instagram with a message that read "I'M BACK" in bold.
"I'll be 38 on Wednesday and I'm feeling just like a fine wine. Getting better with age," Harrison wrote in the message under the photo.
Harrison compiled five sacks and 40 tackles in 15 games (one start) last season as a rotational pass rusher. He has 76.5 career sacks.
The Steelers expected Harrison would return for his 13th NFL season.
Harrison has been undergoing a rigorous six- to eight-week training schedule to ensure he was up for another season, highlighting those workouts daily on social media.
"He's working out awful hard for a retired guy, isn't he?" Tomlin joked.