  I present a 40 kilosecond Nuclear Spectroscopic Telescope Array (NuSTAR)
observation of the recently identified low-luminosity X-ray binary and
transitional millisecond pulsar (tMSP) candidate 1RXS J154439.4-112820, which
is associated with the high-energy gamma-ray source 3FGL J1544.6--1125. The
system is detected up to ~30 keV with an extension of the same power-law
spectrum and rapid large-amplitude variability between two flux levels observed
in soft X-rays. These findings provide further evidence that 1RXS
J154439.4-112820 belongs to the same class of objects as the nearby bona fide
tMSPs PSR J1023+0038 and XSS J12270-4859 and therefore almost certainly hosts a
millisecond pulsar accreting at low luminosities. I also examine the long-term
accretion history of 1RXS J154439.4-112820 based on archival optical,
ultraviolet, X-ray, and $\gamma$-ray light curves covering the past
$\sim$decade. Throughout this period, the source has maintained similar flux
levels at all wavelengths, which is an indication that it has not experienced
prolonged episodes of a non-accreting radio pulsar state but may spontaneously
undergo such a state transformation in the future.
