Koirala travelled to New York's Memorial Sloan-Kettering Cancer Center last week for treatment after routine tests showed up a spot on his lungs, prompting cancer fears.
"The doctors have diagnosed the PM with tuberculosis, not cancer, and have told him that it can be cured quite easily," said Dilendra Prasad Badu, spokesman for the ruling Nepali Congress party.
"I spoke to the PM today, he is feeling good and expects to be back in Nepal within a week to ten days," Badu told AFP in Kathmandu.
Koirala, who was a habitual cigarette smoker until he was diagnosed with tongue cancer eight years ago, will undergo radiation treatment to prevent a recurrence of that illness before returning to Nepal, Badu said.
The prime minister, who heads the Nepali Congress, took office in February after winning elections on a pledge to deliver a long-delayed constitution to the Himalayan nation within a year.