After playing for the St. Louis AAA Blues U16 and committing to the USA Hockey National Team Development Program (USNDPT), Tkachuk agreed to play for Boston University after completing two seasons with the USNDPT. While playing for the university's Boston Terriers team, he had four goals and 14 points in 19 games as a freshman before making the U.S. under-20 national team for the 2018 World Junior Championships. Nearing the end of the season, Tkachuk was selected for the Hockey East Rookie Team after ranking fifth on the team in points.