Some 27 people were in cardiopulmonary arrest, an official at Osaka's city fire department told Reuters, the term used in Japan before a death is officially confirmed. Another person was injured, the official said.
Nine people were confirmed dead at hospital, broadcaster TV Asahi said.
Police were investigating suspected arson, including reports that a man started the fire in the building, Kyodo News said, citing people involved in the investigation.
An elderly man brought a bag into the building that leaked flammable liquid and was ignited, the Yomiuri newspaper said. The man was believed to be a patient at the clinic, the Mainichi newspaper said.
The fire broke out shortly after the clinic, on the fourth floor of a multi-story office building, opened for business at 10 a.m. (0100 GMT) and was largely extinguished within 30 minutes, public broadcaster NHK reported.