Abstract Purpose Prenatal factors such as maternal stress, infection and nutrition affect fetal brain development and may also influence later risk for dementia. The purpose of this systematic review was to provide an overview of all studies which investigated the association between prenatal factors and later risk for dementia. Methods We systematically searched MEDLINE and Embase for original human studies reporting on associations between prenatal factors and dementia from inception to 23 November 2022. Prenatal factors could be any factor assessed during pregnancy, at birth or postnatally, provided they were indicative of a prenatal exposure. Risk of bias was assessed using the Newcastle Ottawa Scale. We followed PRISMA guidelines for reporting. Results A total of 68 studies met eligibility criteria (including millions of individuals), assessing maternal age (N = 30), paternal age (N = 22), birth order (N = 15), season of birth (N = 16), place of birth (N = 13), prenatal influenza pandemic (N = 1) or Chinese famine exposure (N = 1), birth characteristics (N = 3) and prenatal hormone exposure (N = 4). We observed consistent results for birth in a generally less optimal environment (e.g. high infant mortality area) being associated with higher dementia risk. Lower and higher birth weight and prenatal famine exposure were associated with higher dementia risk. The studies on season of birth, digit ratio, prenatal influenza pandemic exposure, parental age and birth order showed inconsistent results and were hampered by relatively high risk of bias. Conclusion Our findings suggest that some prenatal factors, especially those related to a suboptimal prenatal environment, are associated with an increased dementia risk. As these associations may be confounded by factors such as parental socioeconomic status, more research is needed to examine the potential causal role of the prenatal environment in dementia.