The stabbing happened at a Crunch Fitness in the 12800 block of Valley View Avenue just before 5:30 p.m. and prompted a response from the Los Angeles County Sheriff's Department and the county fire department.
According to the Norwalk sheriff's station, the man was stabbed somewhere in the back and was taken to a hospital, where he is in stable condition.
The assailant was only described as a man. He fled the scene, authorities said.
A weapon was not recovered.
Further details about the incident, including a motive, were not immediately available.