Calvin Thomas of Lowell called police after he and his twin 7-year-old daughters spotted the gator Saturday night in the Merrimack Canal, The Sun in Lowell reported Monday.
"They thought I was joking," Thomas said, referring to police dispatchers.
Sgt. Jack Cullen said he was shocked to arrive at the scene and discover the call had not been a prank.
Animal Control Officer Robert Pickard caught the alligator, which appeared to be uninjured, with a net and said the reptile will be taken to an exotic animals expert.
Thomas said the gator sighting saved an evening that had been marred by arguing between his daughters.
"I decided to take them for a walk ... and lo and behold an alligator in the canal," he said. "They got home and fell asleep talking about the alligator -- what a great night."