  We report the detection of 376.05 Hz (2.66 ms) coherent X-ray pulsations in
NICER observations of a transient outburst of the low-mass X-ray binary IGR
J17494-3030 in 2020 October/November. The system is an accreting millisecond
X-ray pulsar in a 75 minute ultracompact binary. The mass donor is most likely
a $\simeq 0.02 M_\odot$ finite-entropy white dwarf composed of He or C/O. The
fractional rms pulsed amplitude is 7.4%, and the soft (1-3 keV) X-ray pulse
profile contains a significant second harmonic. The pulsed amplitude and pulse
phase lag (relative to our mean timing model) are energy-dependent, each having
a local maximum at 4 keV and 1.5 keV, respectively. We also recovered the X-ray
pulsations in archival 2012 XMM-Newton observations, allowing us to measure a
long-term pulsar spin-down rate of $\dot\nu = -2.1(7)\times10^{-14}$ Hz/s and
to infer a pulsar surface dipole magnetic field strength of $\simeq 10^9$ G. We
show that the mass transfer in the binary is likely non-conservative, and we
discuss various scenarios for mass loss from the system.
