John Buccigross of ESPN reported earlier Saturday that Tkachuk informed Boston University coach Albie O'Connell of his intention.
"We had Brady home this weekend and he decided to sign," Keith Tkachuk said. "He is so emotional right now because he loves [Boston University] and his teammates. He's such a great kid. He had the toughest call of his life calling Albie."
The 18-year-old forward, who was selected by the Senators with the No. 4 pick in the 2018 NHL Draft, had 31 points (eight goals, 23 assists) in 40 games as a freshman last season and led BU in assists and shots on goal (131). Tkachuk also helped the United States win the bronze medal at the 2018 IIHF World Junior Championship with nine points (three goals, six assists) in seven games.
While playing with the United States at the World Junior Summer Showcase in Kamloops, British Columbia, last week, Tkachuk (6-foot-3, 196 pounds) said he would make his decision after returning home to St. Louis and speaking with his family. Keith is a United States Hockey Hall of Famer and former NHL forward, and Brady's brother Matthew Tkachuk is a Calgary Flames forward.