We assessed validity and reliability of the new 20m square shuttle run test (SST) for predicting maximal oxygen uptake (V˙O2max) and compared it with its predecessor, the 20m Multistage Shuttle Run Test (MST). In a repeated-measures randomised-block design, 74 healthy adult males performed the SST, the MST and a treadmill test (TT). To assess reliability, 40 of the total 74 volunteers were randomly-selected to perform the SST and MST twice. Unlike the SST (p>0.05), mean predicted V˙O2max (V˙predO2max) from the MST was significantly increased from that measured during the TT (p<0.05). The V˙predO2max from SST and MST correlated with TT V˙O2max at r=0.95 (p<0.001) and r=0.63 (p<0.001), respectively. Prediction error of SST was −0.3±3.3mlkg −1 min −1 with a coefficient of variation of ±3.5%, while the equivalent values for MST were 4.2±7.3mlkg −1 min −1 and ±7.4%. Mean test–retest V˙predO2max did not differ for both SST and MST (p>0.05), while the corresponding test–retest correlation coefficients were r=0.85 (p<0.001) and r=0.72 (p<0.001). Reliability errors in 95% limits of agreement were 0.3±4.8 and 0.6±6.8mlkg −1 min −1 while coefficients of variation were ±5.2% and ±6.8% for the SST and MST, respectively. It is concluded that SST is a more valid proxy than MST for predicting laboratory V˙O2max based on the current procedures, while both tests are sufficiently reliable in healthy male adults.