Abstract137Cs and 90Sr are hazardous to ecological environment and human health due to their strong radioactivity, long half‐life, and high mobility. However, effective adsorption and separation of Cs+ and Sr2+ from acidic radioactive wastewater is challenging due to stability issues of material and the strong competition of protons. Herein, a K+‐activated niobium germanate (K‐NGH‐1) presents efficient Cs+/Sr2+ coadsorption and highly selective Cs+/Sr2+ separation, respectively, under different acidity conditions. In neutral solution, K‐NGH‐1 exhibits ultrafast adsorption kinetics and high adsorption capacity for both Cs+ and Sr2+ (qmCs = 182.91 mg g−1; qmSr = 41.62 mg g−1). In 1 M HNO3 solution, K‐NGH‐1 still possesses qmCs of 91.40 mg g−1 for Cs+ but almost no adsorption for Sr2+. Moreover, K‐NGH‐1 can effectively separate Cs+ from 1 M HNO3 solutions with excess competing Sr2+ and Mn+ (Mn+ = Na+, Ca2+, Mg2+) ions. Thus, efficient separation of Cs+ and Sr2+ is realized under acidic conditions. Besides, K‐NGH‐1 shows excellent acid and radiation resistance and recyclability. All the merits above endow K‐NGH‐1 with the first example of niobium germanates for radionuclides remediation. This work highlights the facile pH control approach towards bifunctional ion exchangers for efficient Cs+/Sr2+ coadsorption and selective separation.