Justiciar is a title employed in the Seven Kingdoms and the Free Cities by civil servants and nobles tasked with dispensing justice.
Prince Doran Martell employs justiciars to sit in judgment in Sunspear while remaining at the Water Gardens.
In Braavos, justiciars are civil servants. Like keyholders, they dress in drab coats of brown and grey.
A Feast for Crows
After the death of Tywin Lannister, Queen Regent Cersei renames the office of Master of laws as "justiciar" in the style of the Free Cities, and grants the post to Orton Merryweather.
During her time as "Cat of the Canals", Arya Stark sells clams and cockles to keyholders and justiciars.
A Dance with Dragons
After assuming the regency, Ser Kevan Lannister restores the title of Master of laws but maintains that of justiciar. He grants the post to Randyll Tarly to amend the damages done by Cersei to the alliance of House Lannister to House Tyrell.