It was a scene that even the most optimistic American University fans could not have envisioned when this basketball season began: a gang of blue-clad Eagles, picked to finish next to last in the Patriot League and featuring one senior and a first-year coach, joining dozens of traveling students in celebrating an NCAA tournament berth.

With a 55-36 victory over top-seeded Boston University on Wednesday at Agganis Arena, AU won its third league title in seven years — and easily its most improbable.

“We knew we had low expectations coming into the season, so we felt we needed to show everyone what we’ve got,” sophomore guard Jesse Reed said. “Having this as the end result feels great, almost surreal.”

Making their first appearance in the final since winning consecutive titles in 2008-09, the second-seeded Eagles (20-12) led the entire game and became the sixth Patriot team in 24 years other than the top seed to claim the trophy.

The Terriers, preseason favorites in their first Patriot season after leaving America East, scored 91 points in each of their previous two tournament games but were bedeviled by the AU defense and wayward shooting (30.8 percent).

Tournament MVP Darius “Pee Wee” Gardner, a 5-foot-9 junior transfer from Stephen F. Austin, had 18 points on 7-for-9 shooting and senior center Tony Wroblicky added 15 points and eight rebounds for the Eagles, who shot 55.9 percent.

They will learn their NCAA opponent and destination Sunday night.

“I felt like Kevin Garnett saying, ‘Anything is possible,’ ” Gardner said. “I just didn’t want to scream it out.”

Maurice Watson Jr., who had a nation-high 17 assists in the semifinals, led the Terriers (24-10) with 11 points and five assists but committed seven turnovers. BU made 1 of 17 three-pointers and 3 of 7 free throws.

“We keep the pressure on and don’t allow them to get easy looks, easy shots,” Wroblicky said. “We did a good job of making them take tough shots and they didn’t shoot well.”

Everything flowed AU’s way in the first half. The Terriers made three of their first 18 field goal attempts. The Eagles were very good defensively — but not that good: BU tossed up several open clunkers and appeared burdened by the occasion.

Marko Vasic and John Schoof made consecutive three-pointers as the Eagles busted out to a 16-7 lead.

Strong early starts have not been an accurate barometer in the series: BU led by 11 at AU and lost by 30; AU led by 10 at BU and lost by nine.

The Terriers found their rhythm. AU’s turnovers accumulated. Gardner ended a 41 / 2-minute rut with a three-pointer.

The Eagles labored in their half-court offense, but Gardner bailed them out with an audacious running semi-hook and then a three-pointer with 10 seconds left as the shot clock was about to expire.

The Eagles led 24-16 at halftime, without the benefit of a free throw or offensive rebound and without a point from Reed, the season’s leading scorer. The Terriers were 6 for 25, four baskets coming on layups or tap-ins.

AU Coach Mike Brennan stuck with Wroblicky, despite a third foul with almost 14 minutes left.

The Eagles stretched the lead to 11 as Reed made his first basket on a difficult drive and then scored in transition after Wroblicky’s blocked shot.

Each time BU threatened to make a pivotal run, AU answered. “We were waiting for that one shot to ignite us,” Terriers Coach Joe Jones said. But “it just seemed to spiral.”