Background & aims The thickness of the adductor pollicis muscle (TAPM) is the only muscle that can be directly measured with a caliper. Recent studies demonstrate that the TAPM could be used as part of nutritional screening, but this has not been systematically reviewed. We aimed to review the validity and reliability of TAPM in identifying malnutrition risk. Methods PubMed, CINAHL and Scopus were systematically searched. Eligible studies were cross-sectional, case-control or cohort studies which recruited adult patients in the hospital, measured the TAPM along with a valid nutritional assessment tool and evaluated the validity of TAPM with univariate, multivariate, discriminative and/or agreement statistics. The Quality Assessment of Diagnostic Accuracy Studies-II was used to evaluate the risk-of-bias. Results Nine out of 39 studies identified were eligible, and all had some risk-of-bias. Subjective Global Assessment (SGA) was used as the criterion standard in all nine studies. The TAPM amongst malnourished patients were significantly different from their counterparts but this may be over-estimated since all measurements of the SGA and TAPM were non-blinded. Concordance between the TAPM and SGA ranged from poor to good (kappa ranged from 0.04 to 0.25; specificity ranged from 97.8% to 100%), and this may be due to the varied cut-off values used amongst the studies. There were also disparities in the measurement instruments and methods. Reliability was not reported. Conclusion More studies are needed to establish the reliability of TAPM measurement and cut-off values before the TAPM can be used as a component of nutritional screening.