The aim of this study is to investigate the perturbation effect of parallel-plate ionization chambers on the buildup dose measurement in transverse magnetic fields, using Monte Carlo (MC) simulation. The NACP-02 and ROOS parallel-plate chambers and a PTW31010 cylindrical chamber were modeled for buildup dose measurement in magnetic fields, using the EGSnrc/cavity code. The irradiation condition was set to a 10 × 10 cm2 field in a water phantom at a source-to-surface distance (SSD) of 100 cm, using 6-MV photon spectrum. Magnetic fields of 0 0.35, 1.0, 1.5, and 3.0 T were applied perpendicularly to the direction of the photon beam. The overall perturbation factor PQ,B for the ionization chambers in the magnetic fields was also calculated. The dose to water was enhanced with increasing the magnetic field strength at a depth of less than 1 cm. Over a depth of 1.5 cm, there was no significant difference in the depth doses with and without magnetic field in water. The maximum depth dose (%) for the NACP-02 and ROOS chambers at 1.5 T was higher up to 12% and 14% than the maximum depth dose at 0 T, respectively. The depth dose curves of a PTW31010 chamber have a similar tendency to those of water. The PQ,B values for each chamber were the largest at the phantom surface. The transverse magnetic field has a greater effect on the dose response of the NACP and ROOS chambers than that of the PTW31010 chamber in the buildup region.