The deal is worth $14.25 million and has a maximum value of $27 million with incentives, a source said.
Trubisky said his decision came down to the Steelers and New York Giants.
"This was the best opportunity for me and my family ... the best way to get back on the field and try to win games ... You've got to go out and earn it," he told Fowler.
The Steelers also agreed to re-sign right tackle Chukwuma Okorafor to a three-year, $29.25 million deal, including $20.5 million in first two years, a source told ESPN's Adam Schefter. And they also are expected to sign veteran center Mason Cole to a three-year deal, a source said.
In Pittsburgh, Trubisky will compete with Mason Rudolph for the starting quarterback position as the franchise begins the post-Ben Roethlisberger era. Roethlisberger retired in January after 18 seasons with the Steelers in which he helped the franchise to two Super Bowl victories and finished his career with the fifth-most passing yards (64,088) in NFL history.
The Steelers also have former first-rounder Dwayne Haskins on their roster, and general manager Kevin Colbert said earlier this month he expected to go into training camp with four quarterbacks on the roster.
Time away from a starting role paid off for Trubisky, who spent last season as the backup quarterback for the Buffalo Bills. He accepted a one-year, $2.5 million contract to play behind starter Josh Allen while learning from quarterbacks coach Ken Dorsey and offensive coordinator Brian Daboll, who is now the New York Giants' head coach.
While his only playing time came once games were out of reach, Trubisky, the former starter for the Chicago Bears, got an opportunity to start fresh in a new environment.
"Going to Buffalo really opened my eyes," Trubisky told Fowler in February. "... [Being] in Buffalo and [having] a different way of doing things, you learn what's possible. It helped me get back to instinctual football and using my talents rather than overthinking."
Trubisky, the second overall pick in the 2017 NFL draft, started 50 games in his four seasons with the Bears, finishing with a 29-21 record. He has completed 64.1% of his career passes with 64 touchdowns to 38 interceptions. He also has 1,081 career rushing yards and nine rushing TDs.
Trubisky, 27, had an up-and-down career with the Bears. He made the Pro Bowl in 2018 and then was benched by then-coach Matt Nagy in 2020 after failing to meet lofty expectations.
He found success in the Bills' preseason game against the Bears in 2021, finishing 20-of-28 for 221 yards and a touchdown.
ESPN's Alaina Getzenberg and Brooke Pryor contributed to this report.