  We report the first spectroscopic detection of ethyl cyanide (C$_2$H$_5$CN)
in Titan's atmosphere, obtained using spectrally and spatially resolved
observations of multiple emission lines with the Atacama Large
Millimeter/submillimeter array (ALMA). The presence of C$_2$H$_5$CN in Titan's
ionosphere was previously inferred from Cassini ion mass spectrometry
measurements of C$_2$H$_5$CNH$^+$. Here we report the detection of 27
rotational lines from C$_2$H$_5$CN (in 19 separate emission features detected
at $>3\sigma$ confidence), in the frequency range 222-241 GHz. Simultaneous
detections of multiple emission lines from HC$_3$N, CH$_3$CN and CH$_3$CCH were
also obtained. In contrast to HC$_3$N, CH$_3$CN and CH$_3$CCH, which peak in
Titan's northern (spring) hemisphere, the emission from C$_2$H$_5$CN is found
to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly
different chemistry for this species, consistent with a relatively short
chemical lifetime for C$_2$H$_5$CN. Radiative transfer models show that most of
the C$_2$H$_5$CN is concentrated at altitudes 300-600 km, suggesting production
predominantly in the mesosphere and above. Vertical column densities are found
to be in the range (2-5)$\times10^{14}$ cm$^{-2}$.
