A search and rescue operation is underway for a Darwin man who jumped from a bridge into water below last night.

Brendon Doherty, aged 55, jumped off the Channel Island Bridge into the water for a swim with some mates from work.

He began drifting downstream at 9:42pm, just a few minutes after he was seen laughing and swimming in the water.

He was unable to make it to the shore, and was last seen in the middle of the channel drifting towards Darwin City.

Northern Territory emergency crews said his friends tried to encourage him to swim to shore, but he was unable to in the strong tide. 

Witnesses saw him for about two minutes before they lost sight of him.

He was last seen wearing steel cap boots, shorts and a dark shirt.

The Search and Rescue Section was deployed overnight and has done an extensive search of the Middle Arm area.

Anyone who may have witnessed a male floating or along the shore in the harbour to call Triple Zero immediately and attempt to provide aid.