The atmospheres of low-temperature brown dwarfs and gas giant planets are expected to contain the phosphine molecule, PH3. However, previous observations have shown much lower abundances of this molecule than predicted with atmospheric chemistry models. We report JWST spectroscopic observations of phosphine in the atmosphere of the brown dwarf Wolf 1130C. Multiple absorption lines due to phosphine have been detected around 4.3 micrometers, from which we calculated a phosphine abundance of 0.100 ± 0.009 parts per million. This abundance is consistent with disequilibrium atmospheric chemistry models that reproduced the phosphine abundances in Jupiter and Saturn and is much higher than abundances previously reported for other brown dwarfs or exoplanets. This difference may be related to the low abundance of elements heavier than helium in Wolf 1130C.