A vehicle program rewriting system includes a first control unit configured to execute a first control program for controlling a vehicle, a second control unit configured to execute a second control program for controlling the vehicle, a first communication line that connects, when rewriting at least one of the first and second control programs, the first control unit and a program update unit which transmits program update information to the first control unit, and a second communication line branching from the first communication line and connected to the second control unit. The second control unit executes control of, when the program update information for updating the second control program is transmitted, acquiring the program update information for the second control program from among communication contents between the program update unit and the first control unit acquired via the second communication line.