Wnt-5a has been shown to influence the metastatic behavior of human breast cancer cells, and the loss of Wnt-5a expression is associated with metastatic disease. We show here that NFAT1, a transcription factor connected with breast cancer metastasis, is activated by Wnt-5a through a Ca²⁺ signaling pathway in human breast epithelial cells. This activation was simultaneously counteracted by a Wnt-5a-induced Yes/Cdc42 signaling pathway. The observation that inhibition of the Wnt-5a/Yes/Cdc42 signal prolonged the duration of ionomycin-induced NFAT1 activation revealed the general importance of this pathway. The Wnt-5a-induced inhibition of NFAT1 did not require glycogen synthase kinase 3β, JNK, or Pak1 activity or modulation of the cytoskeleton. Instead, we observed that Wnt-5a induced a complex formation of NFAT1/casein kinase 1α, even upon treatment with ionomycin, which was blocked upon inhibition of the Wnt-5a/Yes/Cdc42 signaling pathway. Our results explain why Wnt-5a/Ca²⁺-induced NFAT activity is hard to detect and suggest a novel mechanism by which Wnt-5a can suppress tumor-specific, agonist-induced NFAT activity and thus the metastatic behavior of breast cancer cells.