144 Background: The incidence of young onset colorectal cancer (yoCRC) is rising at alarming rates. The gut microbiome may be a factor accounting for the increase. We analyzed differences in the intratumoral microbiome of yoCRC vs average onset CRC (aoCRC) and its clinical impact. Methods: We identified 314 histologically confirmed cases of stage I-IV CRC that underwent surgical resection at our institution from 2000-2020, diagnosed <50 years of age for yoCRC and >60 years for aoCRC, who consented to a prospective biorepository. 36 cases were excluded due to nonmalignant, non-adenocarcinoma or metastatic site specimens. Fresh frozen tissue from the primary tumor with paired adjacent nonmalignant tissue specimens were analyzed. 16S rDNA was isolated and sequence reads were assigned to genus level amplicon sequence variants in DADA2 and analyzed for alpha and beta diversity using Phyloseq. Statistical tests included analysis of variance (ANOVA), permutational multivariate analysis (PERMANOVA), linear regression, and Wilcoxon test. Differential abundance and correlation analysis were adjusted for sex and ethnicity as confounding factors. Correlation analysis was adjusted with Benjamini Hochberg correction. Clinical differences were analyzed using Fisher's exact test. Results: Of the cohort of 278 patients, 137 had yoCRC (median age 43 years, range 16-49) and 141 had aoCRC (median age 73 years, range 61-95). yoCRC patients were more likely to have stage III or IV disease at presentation (29% vs 14%, p =0.002; 29% vs 18%, p =0.024 respectively), left sided tumors (74% vs 58%, p =0.003) and receive neoadjuvant therapy (29% vs 15%, p =0.004). yoCRC had significantly higher tumor microbial alpha diversity than aoCRC ( p <2.22e−16, Wilcoxon rank-sum test). Beta diversity analysis demonstrated significantly different diversity of genera between the groups (R2=0.12, p =0.001, PERMANOVA). The prevalent taxa identified in both groups were Lactobacillus, Bacillus and Listeria. Differential abundance analysis (ANOVA, p <0.05) revealed a significant variation of intratumoral microbiome (Table). Correlation analysis revealed an association of longer overall survival (OS) with the presence of Akkermansia in yoCRC (R2 =0.36, p <0.001), but not in aoCRC. Conclusions: We found significant differences between the intratumoral microbiome of yoCRC and aoCRC. In particular, Akkermansia, considered a healthy gut microbe, was found in greater relative abundance in yoCRC and correlated with improved OS. Further studies are warranted to understand the nature of association of these microbes with the development of and outcomes in yoCRC. [Table: see text]