High-resolution infrared spectra of diacetylene (HC4H) are recorded in the 3μm CH stretch region using continuous wave cavity ring-down spectroscopy (cw-CRDS). The HC4H molecules are generated in a pulsed and supersonic planar plasma expansion by discharging a C2H2/He/Ar gas mixture. The jet-cooling realizes a low rotational temperature of Trot∼17K. Vibrational temperatures are found to be relatively high, allowing the detection of vibrationally excited HC4H in direct absorption with energies up to 1800cm−1. In total, sixteen ro-vibrational bands are identified, where nine of them were not reported before. Detailed and systematic rotational analyses are presented, yielding spectroscopic parameters for a series of vibrational levels over energy regions of 0–1800 and 3300–5100cm−1. Using the observed infrared band intensities, two vibrational excitation temperatures of HC4H in the plasma jet, ∼570(50) K for the ν6, ν7 and ν8 bending vibrations, and ∼125(10) K for the ν9 bending vibration, are deduced.