Abstract Non-rocky sub-Jovian exoplanets in high-irradiation environments are rare. LTT 9779b, also known as Transiting Exoplanet Survey Satellite (TESS) object of interest (TOI) 193.01, is one of the few such planets discovered to date, and the first example of an ultrahot Neptune. The planet’s bulk density indicates that it has a substantial atmosphere, so to investigate its atmospheric composition and shed further light on its origin, we obtained Spitzer InfraRed Array Camera secondary eclipse observations of LTT 9779b at 3.6 and 4.5 μm. We combined the Spitzer observations with a measurement of the secondary eclipse in the TESS bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9779b the first TESS exoplanet (and the first ultrahot Neptune) with evidence of a spectral feature in its atmosphere. We did not find evidence of a thermal inversion, at odds with expectations based on the atmospheres of similarly irradiated hot Jupiters. We also report a nominal dayside brightness temperature of 2305 ± 141 K (based on the 3.6 μm secondary eclipse measurement), and we constrained the planet’s orbital eccentricity to e < 0.01 at the 99.7% confidence level. Together with our analysis of LTT 9779b’s thermal phase curves reported in a companion paper, our results set the stage for similar investigations of a larger sample of exoplanets discovered in the hot-Neptune desert, investigations that are key to uncovering the origin of this population.