The Pittsburgh Steelers selected Watt in the first round with the 30th overall pick in the 2017 NFL Draft. Watt was the fourth linebacker selected and the second outside linebacker. He was also the fourth linebacker taken in the first round by the Pittsburgh Steelers since 2013.
On June 14, 2017, the Pittsburgh Steelers signed Watt to a fully guaranteed, four-year, $9.25 million contract with a signing bonus of $4.87 million.
Watt made his professional regular season debut and first career start in the Pittsburgh Steelers' season-opener against the Cleveland Browns on September 10, 2017, where he recorded seven combined tackles, two sacks, and one interception, which came off of quarterback DeShone Kizer, as the Steelers won by a score of 21–18. He recorded his first career sack, tackle, and interception during the game and returned the interception for 17-yards. He became the first rookie to start at linebacker for the Pittsburgh Steelers since Aaron Jones in 1988. The following week, he assisted on two tackles before leaving during the first half of the Steelers' 26–9 victory over the Minnesota Vikings with a groin injury. He was declared out for Week 3 against the Chicago Bears. On October 22, 2017, Watt made six combined tackles and made his fourth sack of the season on Cincinnati Bengals' quarterback Andy Dalton during the Steelers' 29–14 victory. He tied Bud Dupree (2014) and LaMarr Woodley (2007) for the franchise record for most sacks by a rookie with his fourth. Watt finished his rookie season with 54 combined tackles (40 solo), seven pass deflections, seven sacks, and an interception in 15 games and 15 starts.