Hamas announced the move Thursday, the latest in a secretive campaign to rid Gaza of alleged Israeli collaborators.
The campaign has underscored Hamas' deep fears about being penetrated by agents of the Jewish state.
Hamas said its military court sentenced Omar Kawari to death by a firing squad. It provided no other details.
In a separate statement Thursday, Hamas said it had arrested "many" Palestinians in Gaza on suspicion of collaborating with Israel to kill senior members of the enclave's Islamist rulers and bomb training sites and government offices.
Ehab Al-Ghsain, spokesman for Gaza's interior ministry, said some of the suspected collaborators were accused of aiding Israel in a late 2008 war in which 1,400 Palestinians and 13 Israelis were killed.
"The phenomena (collaboration) is small but we have arrested many," Abu Abdallah Lafi, a senior official in Hamas internal security service, told a news conference. He would not say how many people were detained but said they included some women.
Ghsain asserted the suspects posed "a real danger to the unity of the pople and their resistance" against Israel, which has sealed off the narrow coastal enclave by land, sea and air.
In May, Hamas executed three Gazans, convicted as collaborators. Eleven others have also been sentenced to death.
Gazans support crackdowns on accused collaborators but Hamas' latest campaign has also sparked panic because many prominent Gaza residents are believed to have been arrested.
The Associated Press and Reuters contributed to this report