Alligatorweed is well known for its potassium (K+)-accumulating capabilities and its strong resistance to undesired growth conditions. The results of this study revealed properties of K+ accumulation and its contribution to drought stress in alligatorweed. In addition, we attempted to characterize the molecular mechanisms of K+ accumulation in this plant. Alligatorweed plants showed a consistent increase in biomass in response to external K+ concentrations, ranging from micromolar levels up to 50 mmol L−1; K+ was also accumulated accordingly in the plants. The stem was the most K+-accumulating organ, accumulating up to 13% of the K+. Moreover, this K+ superaccumulation caused improved resistance to drought stress. The apparent K+ uptake by the roots showed a typical high-affinity property, and the Michaelis constant increased at higher rates of plant K+ in the starting materials. Furthermore, three putative, K+-uptake transporter complementary DNAs (cDNAs) were isolated from alligatorweed (ApKUP1, ApKUP2, and ApKUP3, respectively) using degenerated primers and rapid amplification of cDNA end techniques. The expression of ApKUP1 and ApKUP3 was predominately localized to the leaves, whereas ApKUP2 was expressed throughout the entire plant. The expression of ApKUP1 and ApKUP3 was stimulated in the stems and roots when K+ was depleted from the external medium. Moreover, ApKUP3 expression was enhanced in the stem in response to abscisic acid treatment and drought stress. In conclusion, our findings provide further insight into the mechanisms of K+ accumulation linked to K+ uptake in alligatorweed.