A marking device for golf balls comprises a seat, an arch-shaped retainer, and a bottom plate. The seat defines a central recess and defines a bottom opening that communicates with the central recess. The seat is provided with an anti-slip pad within the central recess. Also, the seat is provided with upper positioning means and lower positioning means. The arch-shaped retainer is provided with retainer positioning means corresponding to the upper positioning means of the seat. The bottom plate is provided with plate positioning means corresponding to the lower positioning means of the seat. Therefore, the arch-shaped retainer and the bottom plate can be properly positioned on the seat. Furthermore, the arch-shaped retainer defines a central slit and the bottom plate defines a plurality of hollowed-out symbols. As such, the marking device can mark a line or symbol on a golf ball conveniently.