Few details were immediately available about the shooting, which happened at Los Robles Regional Medical Center near the intersection of Janss and Lynn roads.
Spokespersons for the Simi Valley Police Department and Ventura County Sheriff's Office confirmed that both agencies responded to the scene.
The suspect's condition is unknown.
Ventura County sheriff's officials said there is no threat to the public.
"All patients, colleagues and visitors are safe, and our hospital is operating as normal," the hospital said in a statement.
DEVELOPING: More details will be added to this report as they become available.