  We present the analysis of Fermi Large Area Telescope (LAT) $\gamma$-ray
observations of HB~21 (G89.0+4.7). We detect significant $\gamma$-ray emission
associated with the remnant: the flux >100 MeV is
$9.4\pm0.8(stat)\pm1.6(syst)\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$. HB 21 is
well modeled by a uniform disk centered at $l= 88{\deg}.75\pm 0{\deg}.04$, $b =
+4{\deg}.65 \pm 0{\deg}.06$ with a radius of $1{\deg}.19 \pm 0{\deg}.06$. The
$\gamma$-ray spectrum shows clear evidence of curvature, suggesting a cutoff or
break in the underlying particle population at an energy of a few GeV. We
complement $\gamma$-ray observations with the analysis of the WMAP 7-year data
from 23 to 93 GHz, achieving the first detection of HB 21 at these frequencies.
In combination with archival radio data, the radio spectrum shows a spectral
break which helps to constrain the relativistic electron spectrum, hence
parameters of simple non-thermal radiation models. In one-zone models
multiwavelength data favor the origin of $\gamma$ rays from nucleon-nucleon
collisions. A single population of electrons cannot produce both $\gamma$ rays
through bremsstrahlung and radio emission through synchrotron radiation. A
predominantly inverse-Compton origin of the $\gamma$-ray emission is disfavored
because it requires lower interstellar densities than are inferred for HB 21.
In the hadronic-dominated scenarios accelerated nuclei contribute a total
energy of $\sim 3 \times10^{49}$ ergs, while in a two-zone
bremsstrahlung-dominated scenario the total energy in accelerated particles is
$\sim1\times10^{49}$ ergs.
