BACKGROUND: Subjective cognitive decline (SCD) is considered an early preclinical marker for Alzheimer's disease (AD). However, prevalence estimates vary widely due to inconsistent definitions. SCD Plus criteria were proposed by experts to improve specificity for preclinical AD. Population-based evidence on the prevalence and correlates of SCD Plus remains limited. METHODS: Data from three population-based European cohorts (LIFE-Adult-Study, English Longitudinal Study of Ageing, and Cognitive Function and Ageing Study) comprising adults aged ≥ 60 years without dementia or marked cognitive impairment were harmonized to derive a common definition of SCD Plus based on available core criteria. Generalized linear models examined correlates of SCD Plus in pooled and study-specific analyses. RESULTS: Among 18,795 participants (mean age (SD): 72.1 (6.8) years; 55.5% women), prevalence of SCD Plus, based on the harmonized operationalization, was 37.9% (33.3-53.7% across cohorts). In pooled analyses, SCD Plus was associated with higher education, depression, anxiety, hypertension, diabetes, heart disease, Parkinson's disease, and history of stroke, and inversely associated with smoking and better cognitive performance. Study-specific analyses additionally indicated associations with sleep and hearing-related problems, lower physical activity, thyroid disease, and personality traits (higher neuroticism and lower openness, agreeableness, conscientiousness). DISCUSSION: SCD Plus was highly prevalent in three large European cohorts, assessed using a harmonized operationalization approach. Associations with several established, modifiable dementia risk factors underscore the relevance of SCD Plus for clinical risk assessment. Longitudinal studies are needed to determine whether the identified correlates influence subsequent cognitive decline in individuals with SCD Plus.