Khalid Mehdiyev, 27, told jurors he was arrested in July 2022 in his car in Brooklyn, New York City, while in possession of an AK-47 rifle and a ski mask.
"I was there to try to kill the journalist," Mehdiyev said in Manhattan federal court on Tuesday.
Prosecutors say Mehdiyev was hired by the associates Rafat Amirov and Polad Omarov to kill Masih Alinejad, a New York-based journalist who left Iran in 2009 and is known for outspoken criticism of the government in Tehran and its treatment of women.
Amirov, 45, and Omarov, 40, have pleaded not guilty to murder for hire and attempted murder in aid of racketeering.
They could face decades in prison if convicted.
In his opening statement on Tuesday, federal prosecutor Jacob Gutwillig said Iran's government offered to pay Amirov and Omarov $US500,000 to orchestrate Alinejad's murder.
"The defendants were hired guns for the government of Iran," Gutwillig said.
"Masih Alinejad was almost gunned down on the streets of New York City by a hit man sent by the defendants."
Amirov's lawyer, Michael Martin, countered that prosecutors were relying on circumstantial evidence and "the testimony of a murderer and a liar".
Michael Perkins, a lawyer for Omarov, said the evidence would not show his client intended to kill Alinejad.
A representative for Iran's mission to the United Nations did not immediately respond to a request for comment.
On the stand, Mehdiyev testified that he directed murders, kidnappings and extortion plots during a life of crime that began a decade ago in his native Azerbaijan.
He also said he knew the target of the murder plot underlying the case against Amirov and Omarov as "Masih".
Mehdiyev said he was co-operating with prosecutors after pleading guilty to attempted murder and illegal possession of a firearm, and faces a minimum of 15 years in prison for the attempt on Alinejad's life and separate racketeering charges.
Tehran has called separate allegations that four Iranian intelligence officers sought to kidnap Alinejad in 2021 "baseless".
