Judge Qais Hashim al-Shammari was killed with his brother-in-law in a roadside ambush, the sources said on Tuesday.
It was the latest in a series of killings by anti-US fighters seeking to disrupt elections set for Sunday.
Fighters have assassinated Baghdad's provincial governor and the capital's deputy police chief in recent weeks.
Meanwhile, six US soldiers have been killed in two separate incidents around Baghdad.
A US soldier was killed in a bomb blast in Baghdad on Monday, the US army said in a statement on Tuesday.
"A Task Force Baghdad Soldier died of wounds at approximately 7pm (1600 GMT) on 24 January. The soldier sustained the injuries when a patrol was attacked with an improvised explosive device in western Baghdad," the military said.
A car bomb exploded outside the
Australian embassy last week
Also on Monday, five 1st Infantry Division soldiers died and two were injured in a vehicle accident near Khan Bani Saad, north of Baghdad, a US military statement said on Tuesday. One of the injured was in a serious condition.
The deaths raise the number of US soldiers killed in Iraq to 1374 since the March 2003 invasion, 296 of them in non-combat incidents, according to the latest figures from the Pentagon.
In another incident, the Australian defence department said its soldiers shot and injured a man acting "in a suspicious manner" outside the Australian embassy in Baghdad.
The man was shot on Monday after he had parked a vehicle near a barracks housing Australian troops that was targeted last week in a bomb attack.
Two Iraqis were killed and two Australian soldiers wounded in that attack, the department said in a statement.
"The vehicle stopped near the security detachment flats and the man alighted and despite being given repeated warnings the driver continued to act in a suspicious manner and failed to comply with directions," the statement said.
Australian soldiers then opened fire, injuring the man. He was taken from the scene by Iraqi authorities and his condition was not immediately known.
The Australian Defence Force is investigating the incident.