Wael Hammudeh, 30, was killed in the explosion in a camp of the armed wing of Hamas in southern Gaza, medics and witnesses said.
On Thursday two Hamas gunmen were killed and a third seriously wounded in an explosion that rocked an area south of Gaza City used by the armed wing of the Islamist movement that runs Gaza.
Hamas initially claimed the explosion was caused by an Israeli air raid, but later said its members were killed "while carrying out a jihad mission." It gave no further details.".
The blast took place in a Hamas training base, where ammunition and explosives are being held. Hamas members prevented photographers from entering the site.