Pageau's tally off the rebound of a Marc Methot shot from the point broke a 2-2 tie and boosted the Senators to a victory in a game that had been rescheduled from April 15 due to the Boston Marathon bombings.
Pageau assisted on Erik Condra's first-period goal and Jared Cowen also scored for the Senators, who finished with a 25-17-6 record and claimed the seventh seed in the Eastern Conference.
Robin Lehner turned aside 34-of-36 shots, including 15 in the second period. Kyle Turris added an empty-netter and an assist for Ottawa.
The result means Boston (28-14-6) finishes with the fourth seed and will play Toronto in the first round, while the Canadiens gained the second seed and will take on Ottawa.
The result also sent the New York Islanders into the eighth seed -- they will take on top-seeded Pittsburgh.