In a field study, potassium (K) applied as muriate of potash (MOP) significantly reduced methane (CH4) emission from a flooded alluvial soil planted to rice. Cumulative emission was highest in control plots (125.34 kg CH4 ha−1), while the lowest emission was recorded in field plots receiving 30 kg K ha−1 (63.81 kg CH4 ha−1), with a 49% reduction in CH4 emission. Potassium application prevented a drop in the redox potential and reduced the contents of active reducing substances and Fe2+ content in the rhizosphere soil. Potassium amendment also inhibited methanogenic bacteria and stimulated methanotrophic bacterial population. Results suggest that, apart form producing higher plant biomass (both above- and underground) and grain yield, K amendment can effectively reduce CH4 emission from flooded soil and could be developed into an effective mitigation option, especially in K-deficient soils.