A fishing vessel crew reported picking up the 7ft sea mine a mile from the Needles at around 8am on Saturday, the Maritime and Coastguard Agency (MCA) said.
Bomb-disposal experts from the Royal Navy were called in to detonate the device, with warnings issued to ships and public in the area.
Piers Stanbury, of the coastguard agency, said: “From what we can tell from the pictures and the information from the explosive ordnance disposal team, this is most likely an old German wartime sea mine.”
Two years ago, a second world war bomb containing 290lb of “high explosives” found in Portsmouth harbour was towed out to sea and detonated.
And in 2015, a Navy bomb-disposal team carried out a controlled explosion on another wartime mine off the Isle of Wight.
The 1,500lb device, believed to have been dropped from a German aircraft, was dragged up from the seabed by a dredger.
Additional reporting by PA