In a public appearance late Wednesday, Israeli Defense Minister Ehud Barak called Jabari "the military chief of staff of Hamas."
Jabari was born in 1960 in Gaza City. He began as a member of Abbas' Fatah movement, but switched his allegiance to Hamas during his time in Israeli prisons.
After Israel released him in 1995, he worked for a Hamas-run support group for prisoners. In 1998, he was jailed by Palestinian security forces in Gaza for his involvement with the Hamas military wing. Jabari was released two years later, after Israel shelled Gaza prisons as part of its crackdown on a Palestinian uprising.
In 2003, Jabari became the de facto commander of the Hamas military wing after then-chief Mohammed Deif was seriously wounded in an Israeli attack. Jabari survived four attempts by Israel to kill him. In 2004, an air strike on Jabari's house killed his son, Mohammed, a brother, and three other relatives.
Israel withdrew from Gaza in 2005, but still controls its airspace, seacoast, and all but one of its land crossings.
Hamas has ruled Gaza with an iron hand since the 2007 takeover, deepening the political split with Abbas. Repeated attempts at reconciliation have failed.
The takeover deepened the isolation of Hamas and prompted the group to rely increasingly on Iran and Syria. Jabari was instrumental in developing the Hamas military arsenal and the group's networks in Iran, Sudan and Lebanon.
The Hamas founding charter calls for the destruction of Israel, though the group has been grappling with its political direction in recent years.
Jabari was considered close to Hamas hard-liner Mahmoud Zahar, a Gaza strongman. In 2011, Jabari wrote in a Hamas publication: "As long as the Jews occupy our land, they have one thing [in store], death, or they leave the occupied Palestinian territories."