Marcek told the Special Criminal Court in Pezinok, north of the capital Bratislava, that he accepted guilt, an admission that could reduce his sentence from potential life imprisonment.
Marcek was charged with shooting investigative reporter Jan Kuciak, 27, and his fiancee Martina Kusnirova in their house outside Bratislava in February, 2018.
“I am guilty as per article 1,” Marcek told the court when the presiding judge asked the four defendants to make statements on the charges presented by the prosecutor.
Two other defendants, including businessman Marian Kocner, who were charged with ordering the murder, said they were innocent. One defendant, who police said helped Marcek to carry out the murder, did not make a statement at the hearing.
The murder led to large demonstrations against graft which forced prime minister Fico and the country’s police president to step down.
Several senior justice officials later quit after investigations revealed they had been in contact with Kocner, who was the subject of Kuciak’s reporting.
Marcek’s confession was not a total shock as Slovak public television reported police sources as saying last year he had admitted his guilt during questioning.
A fifth suspect, Zoltan Andrusko, confessed in December to facilitating the murder and a court handed him a 15-year prison sentence.