We present a polarimetric study of the RCW121 star-forming region to derive the orientation of the sky-projected magnetic field component traced by the polarization vectors, the morphology of which tends to follow the cloud's structure. Individual polarization-angle values are consistent across the different bands, having a broad distribution towards the RCW121 HII region. We estimate the corresponding magnetic field orientation in the HII region to have a mean value of -12+/-7{deg}. RCW121 shows an elongated shape in the same direction as the magnetic field orientation, which may be evidence that magnetic pressure opposes the HII region expansion. Serkowski's relation was used to determine individual values of the total-to-selective extinction ratio (RV ) distribution and a weighted mean value of RV=3.17+/-0.05. We derive a foreground component of the polarization degree that is consistent with the literature value for this Galactic region.