Hamas secretly elected a member of the radical Islamist movement's armed wing as its new Gaza leader on Monday.
Yihya Sinwar was elected to head the Hamas political office in Gaza, party officials said on condition of anonymity.
An influential military figure, Sinwar will succeed politician Ismail Haniyeh and become the second most important figure in the party after Khaled Mashaal.
Sinwar was held in Israeli jail for more than 20 years until 2011, when he was released along with more than 1,000 other Palestinian prisoners in exchange for Gilad Shalit, an Israeli soldier captured five years earlier.
He has since become a senior figure in the Izzadin al-Qassam Brigades, Hamas's military wing.
Israel's foreign ministry and prime minister's office declined to comment, but the defence ministry body responsible for the Palestinian territories labelled him the head of Hamas's "radical camp".
Sinwar was born in the Khan Younis refugee camp of southern Gaza in 1962 and founded "Majd," one of Hamas's intelligence services.
In 1988, he was arrested by Israel for terrorist activity and eventually sentenced to four life sentences.
Hamas took control of Gaza in 2007, two years after Israel abandoned it. Sinwar remained in jail for another four years, and was released in October 2011 as part of an exchange for Shalit, who was captured in 2006.
Washington accuses Sinwar of pushing for kidnapping more Israeli soldiers as a bargaining chip for Palestinian prisoners.
Hamas currently claims to be holding four Israelis in captivity in Gaza, though two soldiers among them were killed in the 2014 war.
After his release from jail, Sinwar initially made a number of public appearances.
Later, however, he disappeared from public view and was presented in Hamas media as the commander of Qassam's elite units.
On Monday, he appeared ready to step back into the public sphere at a time when Hamas has been holding elections.
The election process, ongoing for months, is shrouded in mystery and it was unclear how Sinwar was appointed and if and when other appointments will be announced.
There was no reference to his appointment on the Hamas website Monday afternoon.
Haniyeh is seen by many observers as the most likely successor to Hamas's overall leader Mashaal, who currently lives in exile. Sinwar, however, could have significant freedom inside Gaza.
Kobi Michael, an analyst and former head of the "Palestinian" desk at Israel's strategic affairs ministry, said the appointment would alarm Israeli politicians.
"He represents the most radical and extreme line of Hamas," he told reporters. "Sinwar believes in armed resistance. He doesn't believe in any sort of cooperation with Israel."