The diverse forms of protein phosphatase 1 in vivo result from the association of its catalytic subunit (PP1c) with different regulatory subunits, one of which is the G‐subunit (G M) that targets PP1c to glycogen particles in muscle. Here we report the structure, at 3.0 Å resolution, of PP1c in complex with a 13 residue peptide (G M [63–75]) of G M. The residues in G M [63–75] that interact with PP1c are those in the Arg/Lys–Val/Ile–Xaa–Phe motif that is present in almost every other identified mammalian PP1‐binding subunit. Disrupting this motif in the G M [63–75] peptide and the M 110 [1–38] peptide (which mimics the myofibrillar targeting M 110 subunit in stimulating the dephosphorylation of myosin) prevents these peptides from interacting with PP1. A short peptide from the PP1‐binding protein p53BP2 that contains the RVXF motif also interacts with PP1c. These findings identify a recognition site on PP1c, invariant from yeast to humans, for a critical structural motif on regulatory subunits. This explains why the binding of PP1 to its regulatory subunits is mutually exclusive, and suggests a novel approach for identifying the functions of PP1‐binding proteins whose roles are unknown.