We report the discovery of HNC5 in TMC-1. Six lines have been found in harmonic relation, with quantum numbers J = 12−11 up to J = 17−16. The lines can be reproduced with the standard frequency relation for linear molecules with B = 1361.75034 ± 0.00033 MHz and D = 32.2 ± 0.7 Hz. The assignment of the carrier to iminopentadienylidene was achieved through examining the possible candidates at a high level of theoretical ab initio calculations. Motivated by the good agreement between the observed B and the calculated value for HNC5, we searched for it in the laboratory and observed the transitions J = 5−4 to 7−6. The derived rotational and distortion constants are 1361.74998 ± 0.00040 MHz and 26.5 ± 5.5 Hz, respectively. Hence, we solidly conclude that the carrier of the lines found in TMC-1 is HNC5. The calculated dipole moment for this species is 7.7 D and the derived column density is (1.3 ± 0.2) × 1010 cm−2. We used the new QUIJOTE data to improve previous observations of HC4NC and found that the abundance ratio HC4NC/HNC5 is 10 ± 2. The abundance ratio of HC5N and its two isomers HC4NC and HNC5 is 500 ± 80 and 5100 ± 800, respectively. These abundance ratios are higher by a factor of ∼10 than those of the equivalent isomers of HC3N. Chemical models reproduce the observed abundances reasonably well when a chemistry similar to that of the smaller species C3HN isomers is adopted. The formation of HNC5 and HC4NC arises from the dissociative recombination with electrons of the cations HC5NH+ and HC4NCH+.