The Joint Rescue Co-ordination Centre in Halifax says around midnight it received a distress call from the Poseidon Princess approximately 100 km offshore from Yarmouth.
Spokesman Cameron Hillier says the centre co-ordinated a response from nearby fishing vessels as well as a Cormorant helicopter and a Coast Guard vessel.
Hillier says three men were located in a lifeboat and a fourth was rescued from the water.
Shawn d'Entremont of Inshore Fisheries, the company the Poseidon Princess fished for, says three of the men were crew members Martin, Lee and Oscar d'Entremont, and that the fourth man was an onboard fisheries observer.
Shawn d'Entremont says all four men were brought to hospital for medical treatment.
He said he did not know what caused the vessel to capsize but expected there will be an investigation into the incident.