A bulk carrier 30 nautical miles southwest of Hodeida came under fire from unknown assailants, but onboard security guards returned fire and the ship and crew were reported safe.
UKMTO said a skiff opened fire before retreating to a larger vessel about 2 nautical miles away with its automatic identification system switched off; authorities are investigating.
No group immediately claimed the attack, though the area is controlled by Iran-backed Houthi rebels, who have threatened to resume strikes on shipping without yet carrying them out.
The incident adds to renewed maritime insecurity off Yemen after a suspected pirate attack on July 1 south of Balhaf and after earlier Houthi attacks during the Gaza war disrupted Red Sea trade.