Known activities of the ubiquitin-selective AAA ATPase Cdc48 (p97) require one of the mutually exclusive cofactors Ufd1/Npl4 and Shp1 (p47). Whereas Ufd1/Npl4 recruits Cdc48 to ubiquitylated proteins destined for degradation by the 26S proteasome, the UBX domain protein p47 has so far been linked exclusively to nondegradative Cdc48 functions in membrane fusion processes. Here, we show that all seven UBX domain proteins of Saccharomyces cerevisiae bind to Cdc48, thus constituting an entire new family of Cdc48 cofactors. The two major yeast UBX domain proteins, Shp1 and Ubx2, possess a ubiquitin-binding UBA domain and interact with ubiquitylated proteins in vivo. Deltashp1 and Deltaubx2 strains display defects in the degradation of a ubiquitylated model substrate, are sensitive to various stress conditions and are genetically linked to the 26S proteasome. Our data suggest that Shp1 and Ubx2 are adaptors for Cdc48-dependent protein degradation through the ubiquitin/proteasome pathway.