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99Tc contamination at legacy nuclear sites is a serious and unsolved environmental issue. The selective remediation of 99TcO4− in the presence of a large excess of NO3− and SO42− from natural waste systems represents a significant scientific and technical challenge, since anions with a higher charge density are often preferentially sorbed by traditional anion‐exchange materials. We present a solution to this challenge based on a stable cationic metal‐organic framework, SCU‐102 (Ni2(tipm)3(NO3)4), which exhibits fast sorption kinetics, a large capacity (291 mg g−1), a high distribution coefficient, and, most importantly, a record‐high TcO4− uptake selectivity. This material can almost quantitatively remove TcO4− in the presence of a large excess of NO3− and SO42−. Decontamination experiments confirm that SCU‐102 represents the optimal Tc scavenger with the highest reported clean‐up efficiency, while first‐principle simulations reveal that the origin of the selectivity is the recognition of TcO4− by the hydrophobic pockets of the structure. Capturing radioactive species: A robust 3D cationic metal‐organic framework material, SCU‐102, exhibits fast sorption kinetics, large uptake capacity, high distribution coefficient, and a record‐high TcO4− uptake selectivity. The material offers a solution to the long‐term challenge of 99Tc decontamination from complex natural water systems at legacy nuclear sites. [ABSTRACT FROM AUTHOR]