Given the rapid increase in the prevalence of chronic diseases in aging populations, this prospective study including 17 707 adults aged >= 45 years from China Health and Retirement Longitudinal Study was used to estimate the associations between chronic disease, multimorbidity, and depression among middle-aged and elderly adults in China, and explore the mediating factors. Depressive symptoms were assessed using the 10-item Centre for Epidemiological Studies Depression Scale (CES-D-10) questionnaire. Twelve chronic physical conditions, including hypertension, diabetes, dyslipidemia, cancer, chronic lung disease, liver disease, heart failure, stroke, kidney disease, arthritis or rheumatism, asthma, digestive disease were assessed. The prevalence rates for physical multimorbidity and depression (CES-D-10 >= 10) were 43.23% and 36.62%, respectively. Through multivariable logistic models and generalized estimating equation (GEE) models, we found all 12 chronic physical conditions, and multimorbidity were significantly associated with depression. Both mobility problems and chronic pain explained more than 30% of the association for all chronic conditions, with particularly high percentages for stroke (51.56%) and cancer (51.06%) in mobility problems and cancer (53.35%) in chronic pain. Limited activities of daily living (ADL) explained 34.60% of the stroke-cancer relationship, while sleep problems explained between 10.15% (stroke) and 14.89% (chronic lung disease) of the association. Individuals with chronic diseases or multimorbidity are significantly more likely to be depressed. Functional symptoms involving limitations of ADL and mobility difficulties mediated much of the association between chronic diseases and incident depression. These symptoms could be targeted for interventions to ameliorate the incidence of depression among individuals with chronic conditions.