The court also fined the newspaper 25,000 Singapore dollars ($16,400).
Justice Tay Yong Kwang ruled Tuesday against the newspaper and two of its editors, three weeks after Attorney General Walter Woon argued the editorials published in June and July questioned the judiciary's independence from Prime Minister Lee Hsien Loong and the ruling People's Action Party. Not meting out punishment in this case would undermine the country's rule of law, the court said.
The letter to the editor was written by Chee Soon Juan, head of the opposition Singapore Democratic Party.
The editorials and the letter "contained insinuations of bias, lack of impartiality and lack of independence and implied that the judiciary is subservient to Mr. Lee and/or the PAP and is a tool for silencing political dissent," Tay wrote in the ruling.
"There can be no doubt that allegations of the nature mentioned above would immediately cast doubts on the judiciary in Singapore and undermine public confidence."
The newspaper's lawyer, Philip Jeyaretnam, was not immediately available for comment. The Wall Street Journal is published by Dow Jones & Co., a part of News Corp.
Singapore's leaders have sued journalists and political opponents several times in past years for alleged defamation. They have won lawsuits and damages against Bloomberg, the Economist and the International Herald Tribune.
Human Rights Watch called on Singapore last month to stop using defamation lawsuits to stifle criticism and bankrupt opposition politicians, citing the High Court's decision in October to order Chee and his party to pay $416,000 to Lee and his father, Lee Kuan Yew, in damages stemming from a 2006 defamation case.
Government leaders justify suing political opponents, saying it is necessary to defend their personal and professional reputations since it bears on their ability to govern properly and command respect from Singaporeans.