Pin1 regulates the levels and functions of phosphoproteins by catalyzing phosphorylation-dependent cis/trans isomerization of peptidyl-prolyl bonds. Previous Pin1 inhibitors contained phosphoamino acids, which are metabolically unstable and have poor membrane permeability. In this work, we report a cell-permeable and metabolically stable nonphosphorylated bicyclic peptide as a potent and selective Pin1 inhibitor, which inhibited the intracellular Pin1 activity in cultured mammalian cells but had little effect on other isomerases such as Pin4, FKBP12, or cyclophilin A.