Background <p>Childhood sexual abuse (CSA) is an adverse childhood experience that may be associated with long-term physical and mental health. Evidence from China remains limited, and the relationship between self-reported CSA-related experiences and chronic somatic diseases in adulthood has not been fully characterized.</p> Methods <p>We analysed data from the 2023 Psychology and Behavior Investigation of Chinese Residents (PBICR), a national cross-sectional survey. CSA-related exposure was assessed using four self-reported items referring to experiences before 18 years of age. Affirmative responses were summed as an item-count CSA score. For exploratory subtype analyses, the first three items were operationally described as non-penetrative or attempted CSA, whereas the fourth item was described as completed penetrative CSA. Chronic somatic diseases were based on self-reported physician diagnoses. Multivariable logistic regression models estimated odds ratios (ORs) and 95% confidence intervals (CIs), with sequential adjustment for sociodemographic characteristics and co-occurring non-sexual adverse childhood experiences. Sensitivity analyses addressed sex heterogeneity, high-exposure categorization, sparse outcomes, and multiple testing.</p> Results <p>Among 30,054 participants, 5.49% reported at least one CSA-related item. Associations generally increased across CSA scores 0 to 3, whereas estimates for score 4 were attenuated for several outcomes and should not be interpreted as evidence of a strictly monotonic dose-response pattern. After adjustment for sociodemographic factors and non-sexual ACEs, CSA-related exposure remained associated with multimorbidity and several disease-specific outcomes, although some estimates were imprecise. Sensitivity analyses using sex-stratified models, a collapsed high-exposure category (scores 3–4), and penalised logistic regression for sparse outcomes supported the direction of the main findings but also highlighted measurement uncertainty around the completed penetrative item.</p> Conclusions <p>Self-reported CSA-related experiences were associated with higher odds of several chronic somatic diseases and multimorbidity among Chinese adults. Causality cannot be inferred from these data. The cumulative CSA score should be interpreted as an item-count exposure rather than a validated severity scale, and findings involving the completed penetrative item require particular caution because of possible differential interpretation and sparse exposed cases.</p>