AbstractMultimorbidity—the co‐occurrence of two or more chronic health conditions—affects > 86% of people with dementia. It is associated with cognitive and functional decline, reduced health‐related quality of life, increased health‐care use, and higher mortality. The relationship between multimorbidity and dementia is potentially bidirectional; conditions such as hypertension and diabetes increase the risk of developing dementia, and cognitive impairment can complicate their management. This complexity presents challenges in health care and research, affecting treatment decisions and often leading to the exclusion of these individuals from clinical trials. Understanding multimorbidity through long‐term prospective studies is crucial to clarify its relationship with dementia. Investigating specific disease combinations, environmental and genetic factors, and their impacts on cognitive health will guide the development of effective prediction models and inclusive intervention strategies for diverse global populations across the life course.Highlights Multimorbidity affects > 86% of individuals with dementia, worsening outcomes. The relationship between multimorbidity and dementia is potentially bidirectional. Chronic conditions hinder dementia management and clinical trial inclusion. Life‐course multimorbidity research is key to dementia risk reduction strategies. Prospective studies are needed to improve prediction models and interventions.