Despite associations between gut bacterial and viral alterations and colorectal cancer (CRC), the role of the fungal microbiota in CRC remains largely uncharacterized. In a new study, faecal metagenomic sequences from patients with CRC (n = 184) or colorectal adenomas (n = 197) and healthy controls (n = 204) from Hong Kong were analysed. Principal component analysis revealed distinct clusters of patients with CRC or controls, and defined mycobiomes corresponding to early-stage and late-stage CRC. Biomarkers for 14 fungal species enabled the distinction of CRC from controls with an area under the receiver-operating characteristic curve (AUC) of 0.93. Validated AUCs of 0.82 and 0.74 were also obtained for independent cohorts from China and Europe, respectively. Furthermore, ecological analyses revealed synergistic intrafungal and antagonistic bacterial–fungal interactions in colorectal carcinogenesis. Taken together, these findings suggest a role for the gut mycobiota in CRC.