Mu Sochua, the former minister for women's affairs, sat quietly in the court Tuesday as the judge ruled she had defamed Hun Sen at an April news conference.
The suit was brought against her in a tit-for-tat action after she initially attempted to sue the prime minister, also for defamation, over derogatory remarks he allegedly made.
However, the court dismissed her suit. Many rights activists had expected Tuesday's ruling, as there is a history of political interference in Cambodia's judiciary.
Mu Sochua says that outside influence frightened lawyers away from taking her case and affected some of the court's rulings.
"If the court, the judiciary system in Cambodia were free, independent and impartial, my case should not even go to court. It's the case that I filed against the prime minister of Cambodia for defaming me that should have gone to court," she said.
She and human rights activists say the courts are being used to silence government critics, after a rash of law suits was brought against the opposition and civic groups.
"I think he feels I am too critical, I am too vocal and his aim is to destroy the opposition," she said. "I think it's about wanting to eliminate the voice of the opposition."
Mu Sochua originally served as a member of Prime Minister Hun Sen's Cabinet, but later left the government and now is a senior member of the opposition Sam Rainsy Party. The U.S.-educated social activist has long worked to help Cambodian women and children, particularly victims of human trafficking and the sex trade. She has been internationally recognized for her human rights work.
The Sam Rainsy Party has indicated it will pay the $4,000. Almost half will go to the state, the balance to the prime minister. Mu Sochua has one month to appeal the verdict.