Titan harbors a dense, organic-rich atmosphere primarily composed of N2 and CH4, with trace amounts of more complex hydrocarbons and nitrogen-bearing species. Deep spectroscopic observations of Titan were obtained in November and December 2019 using the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6 (~211-275 GHz), for the primary purpose of investigating the high-altitude nitrogen chemistry. As a result of the extremely high sensitivity (RMS noise ~ 1 mJy) achieved by these observations, we obtained the first spectroscopic detection of multiple emission lines from the CH3C3N (methylcyanoacetylene or cyanopropyne) molecule in Titan’s atmosphere. Methylcyanoacetylene and other long carbon-chain-bearing molecules (e.g. HC5N, HC7N) have previously been found to be abundant in dense interstellar clouds, but have yet to be definitively detected in planetary atmospheres. The presence of CH3C3N on Titan was previously inferred through detection of its protonated form: C4H3NH+, using the Ion and Neutral Mass Spectrometer (INMS) instrument onboard the Cassini spacecraft, but the predicted atmospheric abundance of the associated (deprotonated) neutral product is not well constrained due to large differences between recent photochemical models. Here we present the measured abundance of CH3C3N in Titan’s middle atmosphere, and compare with INMS and photochemical model results, providing important constraints on the associated production pathways and reaction rate coefficients. The importance of ALMA for future investigations of Titan’s organic inventory and complex atmospheric chemistry is thus demonstrated.