Cancer outcomes have improved with immune checkpoint inhibitor (ICI) treatment; however, less than half of tumors respond. Emerging data show that some responses to ICI depend on the host’s microbiome. Here, we explore a dietary intervention to modify the microbiome and determine the response to ICIs. Following pre-clinical studies showing the benefit of black raspberries, we conducted a human intervention trial called the BEWELL Study (NCT04267874). The intervention increased the abundance of pro-ICI-response microbes (PIRMs). In mouse models, participants’ post-dietary intervention stool led to smaller tumor volumes following treatment with ICIs than pre-dietary intervention samples. One PIRM, Blautia obeum, was sufficient to improve ICI response. These results suggest that black raspberry nectar can modify the human gut microbiome to promote an improved response to ICIs.