Background: WNTLESS (WLS) is a multi‐transmembrane protein that transports Wnt ligands from the Golgi to the cell surface. Although WLS loss‐of‐function experiments in the developing central nervous system reveal phenotypes consistent with defects in WNT1 and WNT3A signaling, data from complementary gain‐of‐function experiments have not yet been reported. Here, we report the phenotypic consequences of WLS overexpression in cultured cells and in the developing chick spinal cord. Results: Overexpression of small amounts of WLS along with either WNT1 or WNT3A promotes the Wnt/β‐catenin pathway in HEK293T cells, while overexpression of higher levels of WLS inhibits the Wnt/β‐catenin pathway in these cells. Similarly, overexpressed WLS inhibits the Wnt/β‐catenin pathway in the developing spinal cord, as assessed by cell proliferation and specification. These effects appear to be Wnt‐specific as overexpression of WLS inhibits the expression of FZD10, a target of β‐catenin‐dependent transcription. Conclusions: Our results show that overexpression of WLS inhibits Wnt/β‐catenin signaling in the spinal cord. As the activation of the Wnt/β‐catenin pathway in the spinal cord requires WNT1 or WNT3A, our results are consistent with a model in which the relative concentration of WLS to Wnt regulates WNT1/3A signaling in the developing spinal cord. Developmental Dynamics 243:1095–1105, 2014. © 2014 Wiley Periodicals, Inc.