Police said the body was found by a fishing vessel half-a-mile south of Brighton Marina at about 10am.
The death is being treated as unexplained and the identity of the woman is not yet known.
A spokesman for Sussex Police said: “At around 10am this morning Sussex Police received a call from the Coastguard reporting a body had been found in the sea by a fishing vessel around half-a-mile south of Brighton Marina.
“Police are continuing to conduct enquiries in order to identify the deceased female.
“At this time the death is being treated as unexplained and the Coroner’s Office has been informed.”