  We report an Atacama Large Millimeter/submillimeter Array 0.88 mm (Band 7)
continuum detection of the accretion disk around SR 12 c, an $\sim$11 $M_{\rm
Jup}$ planetary-mass companion (PMC) orbiting its host binary at 980 au. This
is the first submillimeter detection of a circumplanetary disk around a wide
PMC. The disk has a flux density of $127 \pm14~\mu$Jy and is not resolved by
the $\sim$0.1" beam, so the dust disk radius is likely less than 5 au and can
be much smaller if the dust continuum is optically thick. If, however, the dust
emission is optically thin, then the SR 12 c disk has a comparable dust mass to
the circumplanetary disk around PDS 70 c but is about five times lower than
that of the $\sim$12 $M_{\rm Jup}$ free-floating OTS 44. This suggests that
disks around bound and unbound planetary-mass objects can span a wide range of
masses. The gas mass estimated with an accretion rate of $10^{-11}~M_\odot$
yr$^{-1}$ implies a gas-to-dust ratio higher than 100. If cloud absorption is
not significant, a nondetection of ${}^{12}$CO(3-2) implies a compact gas disk
around SR 12 c. Future sensitive observations may detect more PMC disks at 0.88
mm flux densities of $\lesssim$100 $\mu$Jy.
