The aim of this study was to investigate the effects of guarana supplementation on cognitive performance before and after a bout of maximal intensity cycling and to compare this to an equivalent caffeine dose. Twenty-five participants completed the randomised double-blind crossover trial by performing cognitive tests with one of three supplements, on three different days: guarana (125 mg/kg), caffeine (5 mg/kg) or placebo (65 mg/kg protein powder). After 30 min of rest, participants performed simple (SRT) and choice reaction time (CRT) tests, an immediate word recall test and Bond–Lader mood scale. This was followed by a cyclingV̇O2maxtest, and cognitive tests were then immediately repeated. Guarana supplementation decreased CRT before exercise (407 (sd45) ms) in comparison with placebo (421 (sd46) ms,P= 0·030) but not caffeine (417 (sd42) ms). SRT after exercise decreased following guarana supplementation (306 (sd28) ms) in comparison with placebo (323 (sd32) ms,P= 0·003) but not caffeine (315 (sd32) ms). Intra-individual variability on CRT significantly improved from before (111·4 (sd60·5) ms) to after exercise (81·85 (sd43·1) ms) following guarana supplementation, and no differences were observed for caffeine and placebo (P> 0·05). Alertness scores significantly improved following guarana supplementation (63·3 (sd13·8)) in comparison with placebo (57·4 (sd13·4),P= 0·014) but not caffeine (61·2 (sd12·8)). There were no changes toV̇O2max, immediate word recall or any other Bond–Lader mood scales. Guarana supplementation appears to impact several parameters of cognition. These results support the use of guarana supplementation to possibly maintain speed of attention immediately following a maximal intensity exercise test (V̇O2max).