Potassium (K) is an important component of fine particulate matter (PM2.5) and has been widely used as a tracer for biomass burning around the world. However, this may not be true in Beijing, where the sources of K are much more complicated. The aim of this research is to investigate whether K can be applied as a sole tracer for biomass burning in Beijing. From 2015 to 2016, the concentrations of K across the four seasons were measured by an Xact 625 monitor, which uses X-ray fluorescence, and the concentrations of K+ during two seasons were measured by an in-situ Gas and Aerosol Composition (IGAC), which uses ion chromatography. It was found that the ratios of K/K+ and K+/PM2.5 were close to that of coal combustion, and K exhibited good correlations with trace metals associated with coal combustion (e.g., Pb, As, Se, and Zn). The ratios of K/Pb during the peak of the haze episodes were very stable (around 15.70), suggesting the influence of a major but consistent source. Therefore, it was clear that coal combustion was one of the major sources of K in Beijing. To estimate the major source contributions to K, the ratios of K/Ca and K/Pb were used to represent dust and coal combustion, respectively. From this study, coal combustion was the major source of K (45–69%), followed by biomass burning and dust. However, a seasonality effect was observed, with the highest source contributions coming from coal combustion in winter (69%), biomass burning in autumn (49%), and dust in spring (19%). This research shows that biomass burning would be overestimated in Beijing using K as a sole tracer, since coal is also a major source of the latter.