The deal covers the remainder of this season plus the next, with a $750,000 average annual value.
Harris registered 20 points — five goals and 15 assists — in 38 games during his senior year as captain of the Northeastern University Huskies.
The 21-year-old from Haverhill, Mass., was named the Hockey East conference's best defensive defenceman and was also named to the All-Conference first team this season. He captured the ECAC championship with the Huskies in 2019.
Harris played for the United States in the 2020 world junior championship and reached the quarterfinals.
He was drafted in the third round, 71st overall, by the Canadiens in 2019.