Crews from Newport, Ryde and Shanklin are on scene in the field off the main Newport to Sandown road, between Arreton and Hale Common.
Firefighters are using specialist water rescue and animal rescue equipment to assist the animal from its predicament.
2 appliances, the animal/water rescue unit and a Land Rover 4×4 are in attendance having been called out at around 11:15.
UPDATE @ 13:30 – The horse has been successfully rescued from the water.
A vet is now liaising with the animal’s owners.