Vincent Muscat changed his plea to guilty on Tuesday and was immediately sentenced at the courtroom in the capital Valletta.
Ms Caruana Galizia, an investigative journalist who specialised in probing corruption, was killed by a bomb while driving in October 2017.
Proceedings against brothers Alfred and George Degiorgio, who have maintained their not guilty pleas, will continue. A fourth man, hotelier Yorgen Fenech, is being charged separately.
As the development unfolded, police arrested three men believed to have supplied the device that killed Ms Caruana Galizia. They are set to be charged in court in the coming days.
Ms Caruana Galizia’s killing led to international calls for accountability, as her investigative reporting had targeted the government of then-premier Joseph Muscat with claims that corruption was everywhere in political and business circles in the island nation.
Joseph Muscat, who is not related to the suspect, resigned last year in the wake of the scandal.
Additional reporting by Associated Press