The incident occurred in the Ramabai Nagar area in Ghatkopar on Sunday afternoon, an official said.
He said an eight-year-old girl entered the nullah to retrieve a ball and got stuck, and Shehzad Sheikh, a daily-wage labourer, jumped in to save her.
Sheikh managed to catch hold of the girl and passed her off to another man, but he got stuck in the silt and waste and sank, the official said.
He said the man was brought out with the help of the police and fire brigade and rushed to a hospital, where doctors declared him dead.
The police have registered an accidental death report (ADR), the official said. PTI ZA ARU