JERUSALEM—Israeli Defense Minister Ehud Barak, a chief architect of Israeli defense strategy and a centrist fixture in politics here for decades, said he plans to retire from public life early next year.

Mr. Barak's announcement on Monday was the latest surprise to roil Israeli politics in the wake of the recent conflict in Gaza and ahead of national elections scheduled for Jan. 22.

Prime Minister Benjamin Netanyahu is widely expected to win that vote. But the country's fast-shifting political map, tumultuous social protest movements and unpredictable security situation mean little is certain.

In his current stint as defense minister, Mr. Barak has been viewed as a moderating influence on Mr. Netanyahu. Many Israelis credit the defense minister for ending the conflict with Palestinians in Gaza without a ground invasion. As a result, Mr. Barak appears set to exit politics on something of a high note.

The departure of Mr. Barak, who said he would step down when a new government is formed after the January vote, could usher in a more hawkish defense minister as Israel juggles a decision whether to strike Iran's nuclear facilities.