James Harrison, 39, will back him up, barring a change in developments in practices this week.
Asked about the switch -- Watt had been listed as a second-teamer in August -- coach Mike Tomlin cited recent practices, which featured Watt in the first-team huddle.
"He's been in that position really since Day 0," Tomlin said. "You don't like to anoint people. You like to watch them earn it. We didn't put it on paper. But it won't be like he'll be in a new position when he gets here on Wednesday. He'll be in the same spot in the huddle he was in yesterday."
The Steelers signed Harrison to a two-year, $3.5 million deal in the offseason but wanted Watt and third-year linebacker Bud Dupree to take ownership of the primary pass-rush spots. Pittsburgh has its most depth in years at this spot, with reserves Arthur Moats and Anthony Chickillo combining for 52 games started in their NFL careers.
Watt, 22, the 30th pick in April's draft, recorded 11 tackles, 2 sacks and 1 pass deflection in 4 preseason games. He has maintained a low-key approach about his rise on the depth chart, saying his plan for training camp was to "keep your mouth shut and be a sponge."