Molecular recognition of an aqueous pertechnetate (TcO4-) anion is fundamentally challenging partly due to the charge-diffuse nature of this anion, which hampers design of new technologies for its separation and detection. To address this gap, simple salts of transition metal complexes that undergo a distinct spectroscopic change upon exposure to aqueous anions were explored. The Pt(II) complex [Pt(tpy)-Br]SbF6 (tpy = 2,2';6',2 ''-terpyridine) undergoes a dramatic color change and intense luminescence response upon TcO4- uptake due to concomitant enhancement of Pt center dot center dot center dot Pt interactions. The spectroscopic response was highly selective and quantitative for aqueous TcO4- among other competing anions. Complementary Raman spectroscopy and microscopy techniques, structural determination, and theoretical methods were employed to elucidate the mechanism of this response at the molecular level.