Escherichia coli has three major K⁺ uptake systems, Trk, Kup, and Kdp, which have been studied extensively at near neutral pH. However, the function of these transporters under acidic conditions is not well understood, although growth and survival under acidic conditions are important for bacterial pathogenesis. In this study, we examined the expression and activity of Kdp under acidic conditions and found that the transport activity of Kdp is decreased at low pH and that the expression of kdp is regulated by the internal K⁺ concentration in a pH-independent manner. Consequently, the low activity of Kdp was compensated for by the induction of its elevated expression by low K⁺ accumulation via Kdp at acidic pH.