The spill of the unidentified chemical from the 6,000-gallon container was reported shortly before 9:30 a.m., a spokesperson for the Long Beach Fire Department said.
Eleven civilian ship workers and a firefighter sustained unspecified injuries in the hazardous materials incident. One of the workers and the firefighter were transported to a hospital in unknown condition.
Firefighters, assisted by lifeguard rescue boats, created a 1,000-foot containment boom perimeter in the port's waters to protect the environment, officials said.
The cause of the spill was not immediately known.