Protonated cyanoacetylene (HC₃NH⁺) was discovered in 1994 by Kawaguchi et al. toward TMC-1. The detection was based on two consecutive rotational transitions, J=5–4 and J=4–3, observed with the Nobeyama 45m telescope. Two previous efforts in 1990 to detect HC₃NH⁺, by Turner et al. and Amano et al., had proven to be unsuccessful. The initial detection of the J=5–4 and J=4–3 lines was confirmed in the survey of TMC-1 by Kaifu et al. in 2004 but did not add any more lines. In 2017, Quénard et al. reported the detection of HC₃NH⁺ toward the pre-stellar core L1544 (Lynds 1544) in Taurus via its J=11–10, 10–9, and 9–8 lines.

The related compound, cyanoacetylene, has been detected since 1971 many times and in many environments.