jats:titleAbstract</jats:title> jats:pWe predict that cyanoacetylene (HCjats:sub3</jats:sub>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 (Njats:sub2</jats:sub>), methane (CHjats:sub4</jats:sub>), and hydrogen cyanide (HCN), as described by Swain et al. First, we construct line lists and cross sections for HCjats:sub3</jats:sub>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 jats:italicμ</jats:italic>m, observable by JWST after one transit. This feature persists within the 1jats:italicσ</jats:italic> uncertainty of the retrieved abundances of HCN and CHjats:sub4</jats:sub>. The signal is detectable for stratospheric temperatures ≲600 K and moderate stratospheric mixing (10jats:sup6</jats:sup> cmjats:sup2</jats:sup> sjats:sup−1</jats:sup> ≲ jats:italicK</jats:italic> jats:sub jats:italiczz</jats:italic> </jats:sub> ≲ 10jats:sup8</jats:sup> cmjats:sup2</jats:sup> sjats:sup−1</jats:sup>). Our results also indicate that HCjats:sub3</jats:sub>N is an important source of opacity that future retrieval models should consider.</jats:p>