"I am very happy to be free again," Sawah Abu Saif said on Thursday, according to the AFP news agency.
ARD on Wednesday announced it was closing its office in the Gaza Strip in protest at his arrest.
Saif, 42, was among dozens of people arrested on Saturday, a day after five senior Hamas militants and a little girl were killed in an explosion which the Islamists blamed on the rival Fatah movement.
"Hamas arrested him on the pretext he was involved in the explosion. We assume he has been tortured. Two days ago we heard he was transferred to some hospital for medical treatment," ARD producer in Israel and the Palestinian territories Muhammad Abu Faneh had told AFP.
The explosion was the deadliest Palestinian attack on the Islamist group since it seized power in the Gaza Strip in June 2007, routing forces loyal to Palestinian president Mahmud Abbas, who also heads Fatah.
"All accusations against Sawah Abu Saif are, according to those who know and work with him, entirely unsubstantiated," ARD chairman Fritz Raff said in a statement.