The Wnt signaling pathway is of central importance in embryogenesis, development and adult tissue homeostasis, and dysregulation of this pathway is associated with cancer and other diseases. Despite the developmental and potential therapeutic significance of this pathway, many aspects of Wnt signaling, including the control of the master transcriptional co-activator β-catenin, remain poorly understood. In order to explore this aspect, a diverse immune llama VHH phagemid library was constructed and panned against β-catenin. VHH antibody fragments from the library were expressed intracellularly, and a number of antibodies were shown to possess function-modifying intracellular activity in a luciferase-based Wnt signaling HEK293 reporter bioassay. Further characterization of one such VHH (named LL3) confirmed that it bound endogenous β-catenin, and that it inhibited the Wnt signaling pathway downstream of the destruction complex, while production of a control Ala-substituted complementarity-determining region (CDR)3 mutant demonstrated that the inhibition of β-catenin activity by the parent intracellular antibody was dependent on the specific CDR sequence of the antibody.