The sliding spotlight synthetic aperture radar (SAR) system, as a high-resolution imaging mode, has played a crucial role in vessel detection and tracking over the ocean. Radial velocity is an important indicator for SAR vessel information extraction, which is of great significance for vessel positioning, actual velocity estimation, and target tracking. However, the unique operating mode and complex signal model of sliding spotlight SAR result in a complex coupling between Doppler frequency shift and azimuth offset caused by vessel motion, making traditional Stripmap mode radial velocity estimation methods unsuitable. This letter based on the imagery signal model of moving targets in sliding spotlight SAR mode, establishes the mapping relationship between radial velocity and theoretical Doppler shift. Finally, a vessel radial velocity estimation method for sliding spotlight SAR mode was proposed, combining the single-look complex (SLC) data from real SAR imagery. The proposed method is validated by simulation results and real SAR SLC data of GaoFen-3 (GF-3) sliding spotlight mode. Additionally, the processed results demonstrate that the estimation error has a root-mean-square (rms) value of 0.34 m/s when compared with the information provided by automatic identification system (AIS) data.