State Route 203 is a state highway within Mono County, California, USA. It runs from the Madera-Mono county line (Minaret Summit) to U.S. Route 395 passing through the town of Mammoth Lakes.

Within Mammoth Lakes, State Route 203 is known as both "Minaret Road" and "Main Street." It connects to Lake Mary and the Mammoth Scenic Loop via secondary roads.

The route begins at Minaret Summit at Devils Postpile National Monument. It then heads east to Mammoth Scenic Loop Road, where it continues at shortly enters the town of Mammoth Lakes, where it is known as both "Minaret Road" and "Main Street". It continues east to its end at U.S. Route 395.

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, and do not necessarily reflect current mileage. The entire route is in Mono County.