  We report on the discovery of the new accreting millisecond X-ray pulsar
SRGAJ144459.2-604207 using the SRG/ART-XC data. The source was observed twice
in February 2024 during the declining phase of the outburst. Timing analysis
revealed a coherent signal near 447.8~Hz modulated by the Doppler effect due to
the orbital motion. The derived parameters for the binary system are consistent
with the circular orbit with a period of $\sim5.2$~h. The pulse profiles of the
persistent emission, showing a sine-like part during half a period with a
plateau in between, can well be modelled by emission from two circular spots
partially eclipsed by the accretion disk. Additionally, during our 133~ks
exposure observations, we detected 19 thermonuclear X-ray bursts. All bursts
have similar shapes and energetics, and do not show any signs of photospheric
radius expansion. The burst rate decreases linearly from one per $\sim$1.6~h at
the beginning of observations to one per $\sim$2.2~h at the end and
anticorrelates with the persistent flux. Spectral evolution during the bursts
is consistent with the models of the neutron star atmospheres heated by
accretion and imply a neutron star radius of 11--12~km and the distance to the
source of 8--9~kpc. We also detected pulsations during the bursts and showed
that the pulse profiles differ substantially from those observed in the
persistent emission. However, we could not find a simple physical model
explaining the pulse profiles detected during the bursts.
