The antiproliferative and antitumor activities of 2-hydroxycinnamaldehyde (1), a phenylpropanoid isolated from the bark of Cinnamomum cassia, were investigated using human colorectal cancer cells. Compound 1 exhibited antiproliferative effects in HCT116 colon cancer cells, accompanied by modulation of the Wnt/β-catenin cell signaling pathway. This substance was found also to inhibit β-catenin/T-cell factor (TCF) transcriptional activity in HEK293 cells and HCT116 colon cancer cells. Further mechanistic investigations in human colon cancer cells with aberrantly activated Wnt/β-catenin signaling showed that 1 significantly suppressed the binding of β-catenin/TCF complexes to their specific genomic targets in the nucleus and led to the down-regulation of Wnt target genes such as c-myc and cyclin D1. In an in vivo xenograft model, the intraperitoneal administration of 1 (10 or 20 mg/kg body weight, three times/week) for four weeks suppressed tumor growth in athymic nude mice implanted with HCT116 colon cancer cells significantly, without any apparent toxicity. In an ex vivo biochemical analysis of the tumors, compound 1 was also found to suppress Wnt target genes associated with tumor growth including β-catenin, c-myc, cyclin D1, and survivin. The suppression of the Wnt/β-catenin signaling pathway is a plausible mechanism of action underlying the antiproliferative and antitumor activity of 1 in human colorectal cancer cells.