But U.S. District Judge James Boasberg in the District of Columbia said on Tuesday, Singh, who resigned in May, did not have "head-of-state immunity" from claims arising from his time as India's finance minister.
Inderjit Singh, an Indian Sikh, claimed in the 2013 suit that as finance minister from 1991 to 1996, Manmohan Singh funded cash rewards for members of the military who murdered Sikhs. When he was prime minister, according to the suit, Singh was complicit in the torture and killing of hundreds of thousands of members of the religious minority.
Boasberg said U.S. law bars former heads of state from being sued for actions they took while in office, but not for private acts or those taken in prior government posts.
"While Singh's alleged acts as Finance Minister are not 'private' per se, they did not occur in the course of his official duties as head of state," Boasberg wrote.
Inderjit Singh is representing himself and could not be reached for comment. An attorney the New York City-based human rights group Sikhs for Justice, which helped Inderjit Singh file the suit, did not return a request for comment. Court documents did not list an attorney for Manmohan Singh.
There are more than 21 million Sikhs in India, according to the country's most recent census, but they make up only 2 percent of the population. The group has long had tense relations with India's Hindu majority, and thousands of Sikhs were killed in 1984 in the days following the assassination of Prime Minister Indira Gandhi by her Sikh bodyguards.
Inderjit Singh in his suit claimed Manmohan Singh authorized "counterinsurgency operations" across India in which hundreds of thousands of Sikhs were kidnapped, murdered and buried in mass graves. The prime minister also shielded and promoted political allies and members of the military who orchestrated massacres of Sikhs, Singh said.
The U.S. Department of Justice in May urged Boasberg to dismiss the entire suit because it was filed when Singh was still a sitting head of state. The Justice Department cited a 2004 decision in which the 7th Circuit Court of Appeals dismissed claims against Jiang Zemin, the former president of China, by practitioners of the spiritual discipline Falun Gong.
Boasberg said the cases were distinct because all of the claims against Zemin stemmed from his time as a head of state.
The case is Inderjit Singh v Manmohan Singh, U.S. District Court for the District of Columbia, No. 13-1460.