Background: Older patients are at risk for adverse outcomes after surgical treatment of cancer. Identifying patients at risk could affect treatment decisions and prevent functional decline. Screening tools are available to select patients for Geriatric Assessment. Until now their predictive value for adverse outcomes in older colorectal cancer patients has not been investigated. Objective: To study the predictive value of the Geriatric 8 (G8) and Identification of Seniors at Risk for Hospitalized Patients (ISAR-HP) screening tools for adverse outcomes after elective colorectal surgery in patients older than 70 years. Primary outcomes were 30-day complication rates, secondary outcomes were the length of hospital stay and six-month mortality. Study Design and Methods: Multicentre cohort study from two hospitals in the Netherlands. Frail was defined as a G8 ≤ 14 and/or ISAR-HP ≥ 2. Odds ratio (OR) is given with 95% CI. Results: Overall, 139 patients (52%) out of 268 patients were included; 32 patients (23%) were ISAR-HP-frail, 68 (50%) were G8-frail, 20 were frail on both screening tools. Median age was 77.7 years. ISAR-HP frail patients were at risk for 30-day complications OR 2.4 (CI 1.1–5.4, p = 0.03), readmission OR 3.4 (1.1–11.0), cardiopulmonary complications OR 5.9 (1.6–22.6), longer hospital stay (10.3 versus 8.9 day) and six-months mortality OR 4.9 (1.1–23.4). When ISAR-HP and G8 were combined OR increased for readmission, 30-day and six-months mortality. G8 alone had no predictive value. Conclusions: ISAR-HP-frail patients are at risk for adverse outcomes after colorectal surgery. ISAR-HP combined with G8 has the strongest predictive value for complications and mortality. Key Points: Patients screening frail on ISAR-HP are at increased risk for morbidity and mortality. Screening results of G8 alone was not predictive for postoperative outcomes. Predictive value increased when G8 and ISAR-HP were combined.