Lawyer Adel Mohammed Abbas was killed and his colleague Tamer Hammud Hadi was wounded in the shooting on Tuesday in Baghdad, an Interior Ministry source said.
Lawyer Tamer Hammud Hadi was working for Barzan Ibrahim al-Tikriti, one of the co-defendants of ousted president Saddam Hussein.
Saddam Hussein is on trial over the alleged massacre of more than 140 Shia civilians from the town of Dujail during his time as president.
Barzan, a Saddam half-brother, is defending charges that he shared responsibility for the 1982 massacre.
Barzan once headed the feared Mukhabarat intelligence service.
At 1.30pm (1030 GMT) on Tuesday "armed gunmen fired upon Tamer Hammud Hadi and his colleague Adel Mohammed Abbas" in the Adl neighbourhood of Baghdad, the source said.
Hadi was taken to a north Baghdad hospital.
The trial of Saddam Hussein and his seven co-defendants opened on 19 October.
The trial of Saddam Hussein
opened on 19 October
All pleaded not guilty. If convicted they could be executed.
Detained on 16 April 2003, Barzan was number 52 on the wanted list issued by US commanders after their March 2003 invasion.
He remained one of Saddam's most trusted aides and managed his personal fortune until 1995. He is also believed to have coordinated covert purchases in Europe for Baghdad's prized weapons programmes.
In the northern city of Mosul, an Iraqi journalist working for a local weekly publication was shot dead, police said on Tuesday.
Ahmed Hussein Malek, who worked for the Tall Afar Today, was on Monday leaving an internet cafe in Mosul "when men aboard a car opened fire, killing him on the spot", said Sergeant Omar Muzahem.
Malek was married and had a daughter, a close friend said.
Tall Afar Today is an independent publication that covers events in the largely Sunni region, a stronghold of anti-US fighting.
On 20 October, a top official with the Union of Iraqi Journalists, Mohammed Harun Hassen, was shot dead by unknown men while driving in his car in downtown Baghdad.
According to the rights group Reporters Without Borders, at least 73 journalists and media assistants - overwhelmingly Iraqis - have been killed since the 2003 US-led invasion of Iraq.