Belichick, who was introduced as "the greatest head coach in NFL history," waved the black and gold flag back and forth above his head. Special Olympics Massachusetts athlete James Coffey joined the coach during the pre-game festivities.
After energizing the crowd, Belichick exited the lower bowl. Before leaving, he greeted national anthem singer Todd Angilly.
Later in the second period, the jumbotron in the arena showed Belichick in a private suite with close friends and Patriots staff members, including New England offensive coordinator Josh McDaniels.
Patriots owner Robert Kraft, who also attended Game 1 of the Stanley Cup Final, was shown in a different section in the arena while waving a Bruins rally towel.
Belichick has guided the Patriots to six Super Bowl championships during his tenure as head coach. New England defeated the Los Angeles Rams in Super Bowl LIII last season.
The Bruins and St. Louis Blues were tied 2-2 at the end of the third period. Boston holds a 1-0 lead in the best-of-seven series.