Financial terms were not released.
The 18-year-old defenseman had 41 points (17 goals, 24 assists) in 67 games with Kelowna of the Western Hockey League last season. He led all WHL rookie defensemen in points and goals and all rookies with 182 shots on goal.
RELATED:[2019 NHL Draft first-round pick signings tracker\]
Thomson will represent Finland at the 2019 World Junior Summer Showcase at USA Hockey Arena in Plymouth, Michigan, from July 27-Aug. 3. The four-team showcase, which also includes the United States, Canada and Sweden, starts the process of choosing rosters for the 2020 IIHF World Junior Championship.
"Lassi has great offensive instincts and an NHL-caliber shot," Senators general manager Pierre Dorion said. "He also brings a high compete level, he's always moving his feet and driving the pace of play. We came away from development camp in Ottawa (in June) very impressed and feel that he is going to be an important piece of our team's future."
