Fourteen members of a family, including seven children, were found massacred, and one seriously injured, in a house in Thane early on Sunday, police said.
The incident took place in the city's Kasarvadavli area when the assailant Asnain Anwar Warekar, slit the victims' throats one after the another.
Later, Warekar, 32, committed suicide.
Though the motive behind the massacre is unclear, preliminary inquiries point towards a family dispute, an official said.
The survivor, said to be the assailant's sister, has been rushed to a local hospital where her condition is described as critical.