Abstract Introduction With growing awareness of the importance of rehabilitation, new measures are being developed specifically for use in the intensive care unit (ICU). There are currently 26 measures reported to assess function in ICU survivors. The Physical Function in Intensive care Test scored (PFIT-s) has established clinimetric properties. It is unknown how other functional measures perform in comparison to the PFIT-s or which functional measure may be the most clinically applicable for use within the ICU. The aims of this study were to determine (1) the criterion validity of the Functional Status Score for the ICU (FSS-ICU), ICU Mobility Scale (IMS) and Short Physical Performance Battery (SPPB) against the PFIT-s; (2) the construct validity of these tests against muscle strength; (3) predictive utility of these tests to predict discharge to home; and (4) the clinical applicability. This was a nested study within an ongoing controlled study and an observational study. Methods Sixty-six individuals were assessed at awakening and ICU discharge. Measures included: PFIT-s, FSS-ICU, IMS and SPPB. Bivariate relationships (Spearman’s rank correlation coefficient) and predictive validity (logistic regression) were determined. Responsiveness (effect sizes); floor and ceiling effects; and minimal important differences were calculated. Results Mean ± SD PFIT-s at awakening was 4.7 ± 2.3 out of 10. On awakening a large positive relationship existed between PFIT-s and the other functional measures: FSS-ICU (rho = 0.87, p < 0.005), IMS (rho = 0.81, p < 0.005) and SPPB (rho = 0.70, p < 0.005). The PFIT-s had excellent construct validity (rho = 0.8, p < 0.005) and FSS-ICU (rho = 0.69, p < 0.005) and IMS (rho = 0.57, p < 0.005) had moderate construct validity with muscle strength. The PFIT-s and FSS-ICU had small floor/ceiling effects <11% at awakening and ICU discharge. The SPPB had a large floor effect at awakening (78%) and ICU discharge (56%). All tests demonstrated responsiveness; however highest effect size was seen in the PFIT-s (Cohen’s d = 0.71). Conclusions There is high criterion validity for other functional measures against the PFIT-s. The PFIT-s and FSS-ICU are promising functional measures and are recommended to measure function within the ICU. Trial registration Clinicaltrials.gov NCT02214823. Registered 7 August 2014).