Importance: Older adults with cancer are at risk of overtreatment or undertreatment when decision-making is based solely on chronological age. Although a geriatric assessment is recommended to inform care, the time and expertise required limit its feasibility for all patients. Screening tools offer the potential to identify those who will benefit most from a geriatric assessment. Consensus about the optimal tool to use is lacking. Objective: To appraise the evidence on screening tools used for older adults with cancer and identify an optimal screening tool for older adults with cancer who may benefit from geriatric assessment. Evidence Review: Systematic review of 4 databases (MEDLINE, Embase, CINAHL [Cumulative Index to Nursing and Allied Health Literature], and PubMed) with narrative synthesis from January 1, 2000, to March 14, 2019. Studies reporting on the diagnostic accuracy and use of validated screening tools to identify older adults with cancer who need a geriatric assessment were eligible for inclusion. Data were analyzed from March 14, 2019, to March 23, 2020. Findings: Seventeen unique studies were included, reporting on the use of 12 screening tools. Most studies were prospective cohort studies (n = 11) with only 1 randomized clinical trial. Not all studies reported time taken to administer the screening tools. The Geriatric-8 (G8) (n = 12) and the Vulnerable Elders Survey-13 (VES-13) (n = 9) were the most frequently evaluated screening tools. The G8 scored better in sensitivity and the VES-13 in specificity. Other screening tools evaluated include the Groningen Frailty Index, abbreviated comprehensive geriatric assessment, and Physical Performance Test in 2 studies each. All other screening tools were evaluated in 1 study each. Conclusions and Relevance: To date, the G8 and VES-13 have the most evidence to recommend their use to inform the need for geriatric assessment. When choosing a screening tool, clinicians will need to weigh the tradeoffs between sensitivity and specificity. Future research needs to further validate or improve current screening tools and explore other factors that can influence their use, such as ease of use and resourcing.