It was previously reported that WD repeat domain 5 (Wdr5), a core member of the mammalian Trithorax complex, is a key regulator for the maintenance of embryonic stem (ES) cell pluripotency and somatic cell reprogramming. However, it remains unclear whether other factors are also involved in this process. Here, we show that CTCF is an upstream regulator of Wdr5: It physically associates with Wdr5 and further transcriptionally controls its expression by directly targeting the Wdr5 gene promoter. As a downstream effector, overexpression of Wdr5 can rescue ES cells from growth defects and decreased formation of induced pluripotent stem cells caused by CTCF knockdown. Our results reveal the functional relevance of CTCF and Wdr5, and they connect them to the re-establishment of pluripotency.