The armadillo-related protein β-catenin has multiple functions in cardiac tissue homeostasis: stabilization of β-catenin has been implicated in adult cardiac hypertrophy, and downregulation initiates heart formation in embryogenesis. The protein is also part of the cadherin/catenin complex at the cell membrane, where depletion might result in disturbed cell–cell interaction similar to N-cadherin knockout models. Here, we analyzed the in vivo role of β-catenin in adult cardiac hypertrophy initiated by angiotensin II (Ang II). The cardiac-specific mifepristone-inducible αMHC-CrePR1 transgene was used to induce β-catenin depletion (loxP-flanked exons 3 to 6, β-cat^Δex3–6 mice) or stabilization (loxP-flanked exon 3, β-cat^Δex3 mice). Levels of β-catenin were altered both in membrane and nuclear extracts. Analysis of the β-catenin target genes Axin2 and Tcf-4 confirmed increased β-catenin–dependent transcription in β-catenin stabilized mice. In both models, transgenic mice were viable and healthy at age 6 months. β-Catenin appeared dispensable for cell membrane function. Ang II infusion induced cardiac hypertrophy both in wild-type mice and in mice with β-catenin depletion. In contrast, mice with stabilized β-catenin had decreased cross-sectional area at baseline and an abrogated hypertrophic response to Ang II infusion. Stabilizing β-catenin led to impaired fractional shortening compared with control littermates after Ang II stimulation. This functional deterioration was associated with altered expression of the T-box proteins Tbx5 and Tbx20 at baseline and after Ang II stimulation. In addition, atrophy-related protein IGFBP5 was upregulated in β-catenin–stabilized mice. These data suggest that β-catenin downregulation is required for adaptive cardiac hypertrophy.