Abstract The glycoprotein CD44 is a key regulator of malignant behaviors in breast cancer cells. To date, hyaluronic acid (HA)-CD44 signaling pathway has been widely documented in the context of metastatic bone diseases. Core 1 β1,3-galactosyltransferase (C1GALT1) is a critical enzyme responsible for the elongation of O-glycosylation. Aberrant O-glycans is recognized as a hallmark in cancers. However, the effects of C1GALT1 on CD44 signaling and bone metastasis remain unclear. In this study, IHC analysis indicated that C1GALT1 expression positively correlates with CD44 in breast cancer. Silencing C1GALT1 accumulates the Tn antigen on CD44, which decreases CD44 levels and osteoclastogenic signaling. Mutations in the O-glycosites on the stem region of CD44 impair its surface localization as well as suppress cell–HA adhesion and osteoclastogenic effects of breast cancer cells. Furthermore, in vivo experiments demonstrated the inhibitory effect of silencing C1GALT1 on breast cancer bone metastasis and bone loss. In conclusion, our study highlights the importance of O-glycans in promoting CD44-mediated tumorigenic signals and indicates a novel function of C1GALT1 in driving breast cancer bone metastasis. Implications: Truncation of GalNAc-type O-glycans by silencing C1GALT1 suppresses CD44-mediated osteoclastogenesis and bone metastasis in breast cancer. Targeting the O-glycans on CD44 may serve as a potential therapeutic target for blocking cancer bone metastasis.