Abstract We predict that cyanoacetylene (HC 3 N) is produced photochemically in the atmosphere of GJ 1132 b in abundances detectable by the James Webb Space Telescope (JWST), assuming that the atmosphere is hydrogen dominated and rich in molecular nitrogen (N 2 ), methane (CH 4 ), and hydrogen cyanide (HCN), as described by Swain et al. First, we construct line lists and cross sections for HC 3 N. Then we apply these cross sections and the model atmosphere of Swain et al. to a radiative transfer model in order to simulate the transmission spectrum of GJ 1132 b as it would be seen by JWST, accounting for the uncertainty in the retrieved abundances. We predict that cyanoacetylene features at various wavelengths, with a clear lone feature at 4.5 μ m, observable by JWST after one transit. This feature persists within the 1 σ uncertainty of the retrieved abundances of HCN and CH 4 . The signal is detectable for stratospheric temperatures ≲600 K and moderate stratospheric mixing (10 6 cm 2 s −1 ≲ K zz ≲ 10 8 cm 2 s −1 ). Our results also indicate that HC 3 N is an important source of opacity that future retrieval models should consider.