Boston had a spring visitor who arrived downtown by way of the harbor channel.

A harbor porpoise, the smallest member of the dolphin family in this region, was spotted swimming near the Fort Point neighborhood early Tuesday morning.

In a video posted to Twitter around 8:30 a.m., the dolphin can be seen popping up briefly, its fin poking through the surface of the murky water, before dipping back down again and fading from view, leaving ripples in its wake.

Tony LaCasse, a spokesman for Boston’s New England Aquarium, confirmed that the marine animal was a juvenile harbor porpoise.

LaCasse said the dolphin, which he described as having a blunt face, was probably about 4 feet long. When it becomes a full-grown adult, it will reach only about 5 feet in length.

A second video sent to the Globe showed two porpoises swimming elegantly near ­Lovejoy Wharf, an area by the locks of the Charles River Dam, in the afternoon.

According to the National Oceanic and Atmospheric Administration, female harbor porpoises are slightly larger than males. They have gray backs and white bellies and throats.

Harbor porpoises are known to venture close to the shore all along the East Coast around this time of year to forage for food.

Although they are often difficult to spot — they surface only briefly — harbor porpoises can be found from Fall River to Newburyport, and everywhere in between, LaCasse said.

“In Boston Harbor, they are seen from the Fish Pier in the Seaport to the salt piles all the way up Chelsea Creek,’’ LaCasse said. “They will come in to forage for all kinds of spawning fish and are usually here from early March to mid-May.’’

The porpoises prefer cooler waters, he said, so once temperatures in the harbor hit the 50s, they will probably opt for hunting far from shore.

Steve Annear can be reached at steve.annear@globe.com. Follow him on Twitter @steveannear.