In this Letter, we report the low-temperature spectrum of the 2ν 1 mode of cyanoacetylene near 1.5 µm. We show first that gaseous HC 3 N can be cooled to 20 K in a buffer-gas cell, with rotational temperatures in equilibrium with the cell walls. We then use the wellresolved low-temperature spectrum to report refined values for the 2ν 1 line positions and excited-state B and D rotational constants. The astrophysical implications of these measurements are discussed with regard to future observations of exoplanetary atmospheres, and we propose this band as a potential tool for quantitative cyanoacetylene detection in exoplanetary atmospheres.