The Thames Coastguard has scrambled a team to the scene of this afternoon's incident, approximately a mile off the seaside town of Felixstowe, Suffolk.
One of the people on board the yacht was pulled to safety but another, who is believed to be a woman, is still missing.
A Maritime and Coastguard Agency spokesman said: "Thames Coastguard is this afternoon coordinating a search after a yacht was in collision with a dredger approximately one mile off Felixstowe.
"One person on board the yacht was pulled to safety, however another person is unaccounted for at this time.
"The Felixstowe Coastguard Rescue Team, two lifeboats from Harwich, two pilot boats, a dive boat, the search-and-rescue helicopter from RAF Wattisham and a number of other vessels are currently involved in the search."