Cancer immunotherapy has been successful in patients with different types of cancers, but its efficacy in treating certain types of colorectal cancer (CRC) is limited. The aim of this study was to explore whether sea cucumber polysaccharides (SCP) could impact resistance to anti-programmed cell death-1 (anti-PD1) immunotherapy of CRC and the role of microbiota in mediating their effects. Mice inoculated with immunotherapy resistant CT-26 CRC cells were pretreated with SCP, followed by treatment with/without the anti-PD1 antibody. SCP alone exhibited no inhibitory effect on tumor growth, but they drastically enhanced the efficacy of anti-PD1 treatment, which alone showed minimal effect on tumor development. Compared to anti-PD1 only treatment, a combination of SCP and anti-PD1 increased CD8