This study was undertaken in order to determine the effects of the constant magnetic field (CMF) on the effectiveness of dairy wastewater treatment under anaerobic conditions. It demonstrated that increasing the value of the magnetic flux in the analyzed range resulted in an increasing effectiveness of the fermentation process. Magnets with induction of 0.38 [T] elicited significant changes in the analyzed parameters of methane fermentation. The magnetic field (MF) was observed to positively affect the sedimentation process of anaerobic sludge, reduced COD concentration in the effluent, and biogas production rate. In the most effective variant, the content of suspended matter in the treated wastewaters was reduced by 26.1% and the effectiveness of COD removal increased by 14.0%, compared to the variant without the application of the physical factor. Magnetic induction of 0.431 caused significant changes in the analyzed parameters of the methane fermentation process.