Knee muscle strength (MS) tests are important in the knee injury control process for agility sport (AS) athletes. Test reliability must be known or data are not valid and clinical decisions are compromised. Reliability of one repetition maximum (1RM) single-leg (SL) leg press (LP), knee flexion (KF) and knee extension (KE) strength tests has not been reported in a single cohort. Knowledge of test reliability for both limbs (R, L) is important in case reliability is substantially different between limbs. The aim of this study was to determine the reliability and standard error of measurement (SEM) of the 1RM SL LP, KF and KE tests for both limbs of uninjured AS athletes. It was hypothesised that tests would have good reliability and small SEM for both limbs. This was a between-day (D1/D2) repeated measures study: 13 athletes participated (male n=6; female n=7; age 25.6±5.5 y; height 171.4±8.4 cm; body mass (BM) 71.8±13.4 kg). Tests employed Cybex VR1 equipment. Test order was: LP, KF, KE. Limb order was randomised for D1, repeated D2. Tests followed typical 1RM procedures: warm-up set (1×10), then consecutive trials with incremental load increases at set percent BM (% BM); 1RM defined as the highest load (kg) moved through the required arc-of-motion. Raw data were normalised to% BM: (load moved BM) × 100. Normality of data, (Shapiro-Wilk), systematic error (paired t-test), relative reliability (intraclass correlation coefficient 2,1 (ICC 2,1)), and absolute reliability (SEM;% BM) were assessed. Bonferroni-corrected α was set a priori: 0.008. Mean ±SD D1/D2 differences were (% BM): LP R 4.3±11.3, L 1.9±20.9; KF R 3.0±7.0, L 0.5±2.8; KE R 1.3±5.1, L 3.1±6.5. All data were normally distributed (p≥0.07). There were no differences between D1/D2 values for any test (p≥0.11). The ICC 2,1 was: LP R 0.98, L 0.94; KF R 0.75, L 0.95; KE R 0.87, L 0.78. The SEM was (% BM): LP R 7.3, L 14.2; KF R 4.9, L 1.9; KE R 3.4, L 4.4. There was no systematic error. All tests demonstrated good reliability, although ICCs between limbs for 1RM KF were very different. All tests yielded small SEMs, although SEMs between limbs for 1RM LP and KF were also very different. Knee injury control studies that measure 1RM LP, KF and KE in AS athletes should be sure to determine test reliability and SEM in both limbs to avoid compromised clinical decision-making for either limb.