In a vehicle electronic control system, an electronic control device (ECU) is configured to rewrite a program by writing the update data received from the vehicle master device in a non-volatile memory. Incomplete provisional software is written in a write area of the non-volatile memory in the ECU. The vehicle master device includes: a specific mode determination unit that is configured to determine whether a specific mode for controlling a sequence of a program update is set; and a rewrite instruction unit that is configured to instruct the rewrite target ECU to write the update data under the specific mode when the specific mode determination unit determines that the specific mode is set.