[en] We used the Magellan adaptive optics system to image the 11 Myr substellar companion 1RXS 1609 B at the bluest wavelengths to date (z′ and Y_s). Comparison with synthetic spectra yields a higher temperature than previous studies of $T_{\mathrm{e}\mathrm{f}\mathrm{f}}=2000\pm <!--\; ??\; -->100\mathrm{K}$ and significant dust extinction of $A_V={4.5}_{-<!--\; ???\; -->0.7}^{+0.5}$ mag. Mass estimates based on the DUSTY tracks gives 0.012–0.015 $M_{\odot <!--\; ???\; -->}$, making the companion likely a low-mass brown dwarf surrounded by a dusty disk. Our study suggests that 1RXS 1609 B is one of the ∼25% of Upper Scorpius low-mass members harboring disks, and it may have formed like a star and not a planet out at ∼320 AU.