  We present the discovery, with the Neutron Star Interior Composition Explorer
(NICER), that SRGA J144459.2-604207 is a 447.9 Hz accreting millisecond X-ray
pulsar (AMXP), which underwent a four-week long outburst starting on 2024
February 15. The AMXP resides in a 5.22 hr binary, orbiting a low-mass
companion donor with $M_d>0.1M_\odot$. We report on the temporal and spectral
properties from NICER observations during the early days of the outburst, from
2024 February 21 through 2024 February 23, during which NICER also detected a
type-I X-ray burst that exhibited a plateau lasting ~6 s. The spectra of the
persistent emission were well described by an absorbed thermal blackbody and
power-law model, with blackbody temperature $kT\approx0.9{\rm\,keV}$ and
power-law photon index $\Gamma\approx1.9$. Time-resolved burst spectroscopy
confirmed the thermonuclear nature of the burst, where an additional blackbody
component reached a maximum temperature of nearly $kT\approx3{\rm\,keV}$ at the
peak of the burst. We discuss the nature of the companion as well as the type-I
X-ray burst.
