Using lectin/antibody immunoassays, chemical tools and O-glycoproteomics, we show GALNT7 modifies O-glycoproteins in prostate cancer cells, and using both in vitro and in vivo studies, we further show that GALNT7 drives prostate tumour growth and correlates with cell cycle and immune signalling pathways. Here, GALNT7 is linked to Familial prostate cancer.