  Holographic dark energy (HDE) describes the vacuum energy in a cosmic IR
region whose total energy saturates the limit of avoiding the collapse into a
black hole. HDE predicts that the dark energy equation of the state transiting
from greater than the $-1$ regime to less than $-1$, accelerating the Universe
slower at the early stage and faster at the late stage. We propose the HDE as a
new {\it physical} resolution to the Hubble constant discrepancy between the
cosmic microwave background (CMB) and local measurements. With Planck CMB and
galaxy baryon acoustic oscillation (BAO) data, we fit the HDE prediction of the
Hubble constant as $H_0^{}\!=\, 71.54\pm1.78\,\mathrm{km\,s^{-1} Mpc^{-1}}$,
consistent with local $H_0^{}$ measurements by LMC Cepheid Standards (R19) at
$1.4\sigma$ level. Combining Planck+BAO+R19, we find the HDE parameter
$c=0.51\pm0.02$ and $H_0^{}\! = 73.12\pm 1.14\,\mathrm{km \,s^{-1} Mpc^{-1}}$,
which fits cosmological data at all redshifts. Future CMB and large-scale
structure surveys will further test the holographic scenario.
