The low response rate of immune checkpoint inhibitors (ICIs) is a challenge. The efficacy of ICIs is influenced by the tumour microenvironment, which is controlled by the gut microbiota. In particular, intestinal bacteria and their metabolites, such as short chain fatty acids (SCFAs), are important regulators of cancer immunity; however, our knowledge on the effects of individual SCFAs remains limited. Here, we show that isobutyric acid has the strongest effect among SCFAs on both immune activity and tumour growth. In vitro, cancer cell numbers were suppressed by approximately 75% in humans and mice compared with those in controls. Oral administration of isobutyric acid to carcinoma-bearing mice enhanced the effect of anti-PD-1 immunotherapy, reducing tumour volume by approximately 80% and 60% compared with those in the control group and anti-PD-1 antibody alone group, respectively. Taken together, these findings may support the development of novel cancer therapies that can improve the response rate to ICIs.