Watson scored six goals and added eight assists in 53 games with the Predators during the 2019-20 regular season. He appeared in all four of Nashville's post-season games, but did not register a point.
"Austin Watson is a big, strong winger who will bring a significant presence to our forward group," Pierre Dorion, general manager of the Senators, said in a press release. "He was among Nashville's forward leaders in blocked shots last season, was the team's leader in hits and is someone who is especially physical on the forecheck. We also expect him to be a valuable asset on our penalty kill."
Watson, 28, has two years remaining on a three-year, $4.5-million contract he signed with Nashville on Oct. 31, 2019.
The pick the Senators traded was previously acquired from the Colorado Avalanche.