Multimorbidity refers to the co-occurrence of multiple health conditions in a single individual. The objective of this systematic review was to synthesize and evaluate research on the prevalence of multimorbidity (including both mental and physical health conditions) for people who have experienced homelessness. MEDLINE, EMBASE, PsycINFO, Web of Science, and OpenGrey were searched for relevant studies between 1997 and 2025. Studies were included if the sample consisted of adults in high-income countries, where the exposure was current or former homelessness, and the outcome was multimorbidity including both mental and physical conditions. Random-effects meta-analyses were used to calculate pooled prevalence estimates. The studies were narratively synthesized, and quality assessed. The search retrieved 6043 papers, 30 of which were eligible for inclusion in the review. Most studies recruited participants from specialist homelessness services (n = 21). More than half of the study samples were over 75% male (N = 16). When excluding studies which applied non-probability sampling strategies, the pooled prevalence was 45% (95% CI, 25-66) for multimorbidity. There was a 34% (95% CI, 22-48) pooled prevalence for trimorbidity (co-occurring mental, physical, and substance/alcohol use conditions). High heterogeneity was observed across studies (I2 > 99%). To conclude, multimorbidity is highly prevalent for people who experience homelessness. There is a lack of research on multimorbidity for women who are or have been homeless, and for those who are not accessing specialist homelessness services. These findings demonstrate the need for the integration, collaboration, and co-ordination between services to support the multimorbid health needs of people who experience homelessness.