  The Kilometer Square Array~(KM2A) of the Large High Altitude Air Shower
Observatory (LHAASO) aims at surveying the northern gamma-ray sky at energies
above 10 TeV with unprecedented sensitivity. Gamma-ray observations have long
been one of the most powerful tools for dark matter searches, as e.g.,
high-energy gamma-rays could be produced by the decays of heavy dark matter
particles. In this letter, we present the first dark matter analysis with
LHAASO-KM2A, using the first 340~days of data from 1/2-KM2A and 230~days of
data from 3/4-KM2A. Several regions of interest are used to search for a signal
and account for the residual cosmic-ray background after gamma/hadron
separation. We find no excess of dark matter signals, and thus place some of
the strongest gamma-ray constraints on the lifetime of heavy dark matter
particles with mass between 10^5 and 10^9~GeV. Our results with LHAASO are
robust, and have important implications for dark matter interpretations of the
diffuse astrophysical high-energy neutrino emission.
