  We report the discovery of millihertz quasi-periodic oscillations (mHz QPOs)
from the neutron star (NS) low-mass X-ray binary 4U 1730--22 using the Neutron
Star Interior Composition Explorer (NICER). After being inactive for almost 50
years, 4U 1730--22 went into outburst twice between June and August 2021, and
between February and July 2022. We analyse all the NICER observations of this
source, and detect mHz QPOs with a significance > $4\sigma$ in 35 observations.
The QPO frequency of the full data set ranged between ~4.5 and ~8.1 mHz with an
average fractional rms amplitude of the order of ~2%. The X-ray colour analysis
strongly suggests that 4U 1730--22 was in a soft spectral state during the QPO
detections. Our findings are consistent with those reported for other sources
where the mHz QPOs have been interpreted as the result of a special mode of He
burning on the NS surface called marginally stable nuclear burning (MSNB). We
conclude that the mHz QPOs reported in this work are also associated with the
MSNB, making 4U 1730--22 the eighth source that shows this phenomenology. We
discuss our findings in the context of the heat flux from the NS crust.
