A series of oligoindole foldamers 1 a–dthat are highly fluorescent were prepared by using a biindole derivative as the repeating unit, and their folding and anion‐binding properties were revealed by 1H NMR and fluorescence spectroscopy. The oligoindoles exist in an extended conformation, but adopt a compact helical structure in the presence of an anion. The anion is entrapped inside the tubular cavity of the helical strand, comprising four aryl units per turn, by multiple hydrogen bonds with the indole NHs. These structural features were confirmed by 1H NMR and fluorescence spectroscopy. When folded by anion binding, 1 b–dshow characteristic downfield shifts of the NH signals and upfield shifts of the aromatic CH signals by Δδ=0.1–1.0 ppm. The average chemical shift for all the aromatic signals of 1 a–dis more upfield shifted as the chain lengthens, as anticipated from the degree of overlapped aromatic surfaces in the helical strand. Moreover, 1 a–dare strongly fluorescent in the absence of an anion. Upon binding an anion such as a chloride, the shorter oligoindoles 1 aand blead to negligible change in the emission spectra, whereas the longer ones 1 cand dresult in dramatic changes, that is, large hypochromic and bathochromic shifts (Δλ=65 and 70 nm) of the emission band, confirming the helical folding. The association constants (Ka) between oligoindoles and tetrabutylammonium chloride strongly depend on the chain length; <1 M−1for 1 a, 630 M−1for 1 b, 1.1×105M−1for 1 c, and 2.9×105M−1for 1 din 20 % (v/v) MeOH/CH2Cl2at 24±1 °C. In addition, the association constants of 1 cand 1 dwith other anions such as fluoride, bromide, iodide, azide, cyanide, acetate, and nitrate are determined to be in the order of 103–106M−1under the same conditions.