Khader Adnan, the Islamic Jihad leader, was on Sunday released from jail following a deal on June in which he agreed to end a 56-day hunger strike.
Adnan's strike had galvanised Palestinians behind a "battle of empty stomachs" against Israeli detentions without trial and both sides had feared the his death could hurt a shaky Gaza truce or spur further violence.
Adnan, 37, had been hospitalised in critical condition after refusing food since May 4. Israel freed him in the early hours on Sunday, the agreed date for his release.
Israel had arrested Adnan last July for the 10th time, detaining him without trial under so-called administrative detention, a method Israel says it employs as a security measure to prevent violence.
Adnan, a father of six from the West Bank city of Jenin which launched celebrations when he returned home, had begun an extended hunger strike in 2012 that also led to his freedom.
Israel has sought to prevent hunger strikes by introducing legislation for prisoners to be force fed, but the measure has hit obstacles, such as condemnation by the national doctors' union, which says it contravenes ethical commitments.
Adnan is a known Islamic Jihad figure in the West Bank, territory that Israel captured in a 1967 war, where Palestinians seek an independent state. Like Islamist Hamas, Islamic Jihad opposes peace deals between the Palestinians and Israel and advocates the destruction of Israel.