  We report the discovery of a ring of low surface brightness radio emission
around the Calvera pulsar, a high Galactic latitude, isolated neutron star, in
the LOFAR Two-metre Sky Survey (LoTSS). It is centered at
$\alpha=14\mathrm{h}11\mathrm{m}12.6\mathrm{s}$, $\delta=+79^\mathrm{o}23'15"$,
has inner and outer radii of $14.2'$ and $28.4'$, and an integrated flux
density at 144 MHz of $1.08\pm0.15$ Jy. The ring center is offset by $4.9'$
from the location of the Calvera pulsar. H$\alpha$ observations with the Isaac
Newton Telescope show no coincident optical emission, but do show a small
($\sim20"$) optical structure internal to the ring. We consider three possible
interpretations for the ring: that it is an H~II region, a supernova remnant
(SNR), or an Odd Radio Circle (ORC). The positional coincidence of the ring,
the pulsar, and an X-ray-emitting non-equilibrium ionisation plasma previously
detected, lead us to prefer the SNR interpretation. If the source is indeed a
SNR and its association with the Calvera pulsar is confirmed, then Calvera's
SNR, or G118.4+37.0, will be one of few SNRs in the Galactic halo.
