Troopers from the Bedford barracks say the crash happened Saturday in East St. Clair Township. That's about 70 miles southeast of Pittsburgh, near New Paris.
Although milk isn't a hazardous material, it can kill fish and other aquatic life if it reaches a waterway in high concentrations.
Bedford County emergency management director David Cubbison says crews worked for hours to contain the spill, while Penelec workers were called to fix power lines downed by the crash. The milk spilled about a mile from Dunnings Creek, which did not appear to be affected.
Police have not released the driver's name, but say he was taken to Bedford Memorial Hospital for minor injuries.