Oxoanions such as carboxylates, phosphates, and sulfates play important roles in both chemistry and biology and are abundant on the cell surface. We report on the synthesis and properties of a rationally designed guanidinium-containing oxoanion binder, 1-guanidino-8-amino-2,7-diazacarbazole (GADAC). GADAC binds to a carboxylate, phosphate, and sulfate in pure water with affinities of 3.6 × 104, 1.1 × 103, and 4.2 × 103 M-1, respectively. Like 2-azacarbazole, which is a natural product that enables scorpions to fluoresce, GADAC is fluorescent in water (λabs = 356 nm, λem = 403 nm, ε = 13,400 M-1 cm-1). The quantum yield of GADAC is pH-sensitive, increasing from Φ = 0.12 at pH 7.4 to Φ = 0.53 at pH 4.0 as a result of the protonation of the aminopyridine moiety. The uptake of GADAC into live human melanoma cells is detectable in the DAPI channel at low micromolar concentrations. Its properties make GADAC a promising candidate for applications in oxoanion binding and fluorescence labeling in biological (e.g., the delivery of cargo into cells) and other contexts.