Background: Behavioural and cognitive symptoms in older adults in long-term care arise from shared neurodegenerative and psychosocial processes, and assessing one domain in isolation risks misdiagnosis, unsafe care plans and missed treatment opportunities. This review examined how frequently studies in residential long-term care simultaneously assess cognitive function and behavioural and psychological symptoms of dementia (BPSD), and how the relationship between these constructs is analysed. Methods: Five databases (CINAHL, PsycINFO, Web of Science, PubMed and the Cochrane Library) were systematically searched from inception to March 2026. Eligible observational and interventional studies recruited long-term care residents, used validated measures of cognition and BPSD, and either reported both outcomes together or explicitly analysed their association. Results: Of 263 records screened, 26 studies met the inclusion criteria. Studies that linked cognitive function with behavioural problems, tended to be intervention-oriented rather than observational. Most assessed cognition and BPSD concurrently but treated them as parallel descriptors, focusing on symptom severity, correlates, or intervention effects rather than formal modeling of the cognition–BPSD relationship. Only a minority examined how changes in cognition and BPSD co-varied over time or jointly informed care outcomes, and instruments and analytic approaches were highly heterogeneous. Conclusions: Integrated cognitive–behavioural assessment in long-term care is feasible but not yet routine. Standardised, culturally responsive protocols that link cognitive status with BPSD profiles are needed to support differential diagnosis (dementia, delirium, depression), guide person-centred care planning and monitor response to interventions, thereby optimising quality of life for residents with dementia.