<b><i>Introduction:</i></b> Sarcopenia is highly prevalent in geriatric rehabilitation inpatients; screening using the Strength, Assistance in walking, Rise from a chair, Climb stairs, Falls history questionnaire (SARC-F) has been recommended. This study assessed the diagnostic accuracy of the SARC-F in identifying sarcopenia according to the European Working Group on Sarcopenia in Older People (EWGSOP), EWGSOP2, and Asian Working Group for Sarcopenia (AWGS) definitions in geriatric rehabilitation inpatients. <b><i>Methods:</i></b> REStOring health of acutely unwell adulTs (RESORT) is an observational, longitudinal cohort of geriatric rehabilitation inpatients. The SARC-F was completed for 2 time-points, status at preadmission (1 month before admission) and at admission; a score ≥4 was considered at risk for sarcopenia. Muscle mass (bioelectrical impedance analysis), handgrip strength (handheld dynamometry), and gait speed (4-m walk test) were measured at admission. Diagnostic accuracy was determined by sensitivity, specificity, and area under the curve (AUC). <b><i>Results:</i></b> The sarcopenia prevalence (<i>n</i> = 290, median age 84.0 years [IQR 79.0–89.0], 56.9% female) was 40.3% (EWGSOP1), 25.4% (EWGSOP2), and 38.8% (AWGS). For preadmission and admission status, respectively, the SARC-F identified 67.9 and 82.1% (EWGSOP), 66.0 and 81.0% (EWGSOP2), and 67.5 and 81.6% (AWGS) inpatients at risk for sarcopenia. The SARC-F showed fair sensitivity (67–74%), poor specificity (32–37%), and poor AUC (0.411–0.474) to identify inpatients at risk for sarcopenia at preadmission status, and fair-good sensitivity (79–84%), poor specificity (17–20%), and poor AUC (0.401–0.432) to identify inpatients at risk for sarcopenia at admission, according to EWGSOP, EWGSOP2, and AWGS definitions. <b><i>Conclusion:</i></b> The SARC-F showed poor diagnostic accuracy in identifying sarcopenia in geriatric rehabilitation inpatients. Assessment of sarcopenia is recommended without screening.