For the ultrahigh-resolution airborne stepped-frequency synthetic aperture radar, very large synthetic bandwidth and very long integration time may lead to a 2-D space-variant (SV) motion error when the aircraft flies off the ideally straight trajectory due to the atmospheric turbulence. This new type of error complicates the motion estimation and motion compensation (MOCO). For the motion estimation, we present a jointly 2-D SV motion error estimation method to simultaneously consider the range-variant motion error and the azimuth-variant motion error. For the MOCO, we propose a 2-D SV-MOCO method. The method is implemented through three processing steps: 1) two-step MOCO for the space-invariant motion error and the range-variant phase error; 2) range block-based chirp-z transform (CZT) for the range-variant envelope error; and 3) range block division for the range-dependent azimuth-variant phase error based on the azimuth subaperture method. Finally, processing of simulated data and real data validates the proposed methods.