A boat exploded near Hart Island in the Bronx Saturday night, injuring three and forcing 19 others to swim for their lives as flames engulfed the vessel, according to fire officials and law enforcement sources.

First responders rushed to the fiery scene after the boat burst into flames around 8:30 p.m. and sent 22 terrified passengers plunging into the Long Island Sound, assistant fire chief Mike Meyers told The Post.

Three people — including one in critical condition — were rescued from the water, while the others swam to the nearby 131-acre islet before being transported by the Coast Guard to a boat basin on City Island.

The victims have since been taken to nearby hospitals for evaluation and are all expected “pull through.”

Sources said the fire was sparked by an explosion on the boat, but officials said the cause is still under investigation.

“The only thing I saw was a bunch of smoke while walking down the block,” a witness told The Post, noting the heavy police and fire presence.

“It was very thick heavy black smoke that lasted about a good 10 minutes. I knew it was pretty serious.”

Photos shared on social media captured plumes of black smoke billowing into the night sky near Hart Island, located at the western end of the Sound.

The type of boat wasn’t immediately known, but Meyers said it was a “pretty decent size” to carry nearly two dozen passengers.

The bridge to City Island has since reopened after it was briefly shut down to move victims.