Officers responded to reports of a shooting in the 600 block of West 19th Street around 8:40 a.m., police said.
Police arriving at the scene did not find any victims but learned a short time later that a 16-year-old boy had turned up at a nearby hospital with a gunshot wound.
He is being treated and is expected to make a full recovery, police said. No other injuries were reported.
Meanwhile, Antioch High School, Antioch Middle School, Live Oak High School, and Fremont and Marsh elementary schools were all placed on lockdown as a precaution as police searched the area for the shooter, Antioch Unified School District student support services director Bob Sanchez said.
The lockdown was lifted after about 20 minutes once police had cleared the area, and parents were notified of the incident, Sanchez said.
The boy who was shot was not an Antioch Unified student, he said.
Investigators are not aware of any homes or cars damaged by the gunfire.
Police said no suspects have been located, and the shooting is under investigation.