Older adults with lower socioeconomic status are less likely to be physically active than those with higher socioeconomic status. To inform future intervention development, this review explored: [i] how effective are physical activity interventions at increasing levels of physical activity amongst older adults with lower socioeconomic status?; [ii] what factors are associated with the acceptability of physical activity interventions amongst older adults with lower socioeconomic status?; [iii] what are the implications for developing physical activity interventions for older adults with lower socioeconomic status? This mixed methods systematic review followed PRISMA guidelines. MEDLINE, CENTRAL, Embase, Scopus, Web of Science, PsycINFO, CINAHL, ASSIA and Sports Medicine and Education Index were searched up to May 2023, to identify quantitative, qualitative and mixed methods primary research studies measuring the effectiveness of and/or experiences of physical activity interventions for older adults (aged ≥ 65 years) with lower socioeconomic status. No limits on country were applied. Included studies were assessed for methodological quality using the Mixed Methods Appraisal Tool. Results were synthesised using a results-based convergent synthesis approach with narrative synthesis of quantitative findings and thematic synthesis of qualitative findings. Thirty studies were included. Mixed effects were found for the effectiveness of physical activity interventions, with positive effects for increases in utilitarian walking (i.e. for transport) but not for leisure, mixed effects for objectively measured physical activity and no effects for self-reported total physical activity or muscle strengthening and flexibility activities. Engaging in physical activity interventions was perceived as offering many benefits, social familiarity was important to intervention acceptability and interventions were seen as more acceptable when they were compatible with the lifestyles of older adults with lower socioeconomic status. Future development of physical activity interventions for older adults with lower socioeconomic status should foster social connections, emphasise health benefits of physical activity, hold interventions in locations that are accessible and familiar to older adults with lower socioeconomic status, minimise costs to participants, employ individuals who share participant characteristics to lead interventions, and combine physical activity with other activities older adults with lower socioeconomic status already do to make more efficient use of time. PROSPERO CRD42023417312; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=417312 .