Background: To synthesise the evidence for the effectiveness of inpatient rehabilitation treatment ingredients (versus any comparison) on functioning, quality of life, length of stay, discharge destination, and mortality among older adults with an unplanned hospital admission. Methods: A systematic search of Cochrane Library, MEDLINE, Embase, PsychInfo, PEDro, BASE, and OpenGrey for published and unpublished systematic reviews of inpatient rehabilitation interventions for older adults following an unplanned admission to hospital from database inception to December 2020. Duplicate screening for eligibility, quality assessment, and data extraction including extraction of treatment components and their respective ingredients employing the Treatment Theory framework. Random effects meta-analyses were completed overall and by treatment ingredient. Statistical heterogeneity was assessed with the inconsistency-value (I2). Results: Systematic reviews (n = 12) of moderate to low quality, including 44 non-overlapping relevant RCTs were included. When incorporated in a rehabilitation intervention, there was a large effect of endurance exercise, early intervention and shaping knowledge on walking endurance after the inpatient stay versus comparison. Early intervention, repeated practice activities, goals and planning, increased medical care and/or discharge planning increased the likelihood of discharge home versus comparison. The evidence for activities of daily living (ADL) was conflicting. Rehabilitation interventions were not effective for functional mobility, strength, or quality of life, or reduce length of stay or mortality. Therefore, we did not explore the potential role of treatment ingredients for these outcomes. Conclusion: Benefits observed were often for subgroups of the older adult population e.g., endurance exercise was effective for endurance in older adults with chronic obstructive pulmonary disease, and early intervention was effective for endurance for those with hip fracture. Future research should determine whether the effectiveness of these treatment ingredients observed in subgroups, are generalisable to older adults more broadly. There is a need for more transparent reporting of intervention components and ingredients according to established frameworks to enable future synthesis and/or replication. Trial registration: PROSPERO Registration CRD42018114323.