Israeli naval gunboats surrounded the four fishermen's fishing boat in the sea and arrested them, Nizar Ayyash, chairman of Gaza's fishermen association, told Xinhua.
The four fishermen belong to the same family, Ayyash said. Israeli naval forces usually free detained fishermen after questioning them for several hours.
Israel says it does so to fight smuggling of weapons to the enclave through the sea.
At the end of a 50-day Israeli offensive on the Gaza Strip, which started July 8, Israel allowed fishermen in the coastal enclave to go for fishing in a six-mile area.
During the large-scale air, sea and ground offensive on the Gaza Strip, Palestinian officials in the health ministry said that 2,140 Palestinians were killed and 11,100 injured, two thirds of them civilians.