The bioactive compounds found in tea from Camellia sinensis, namely theanine, caffeine, and polyphenols, can potentially improve short-term and long-term health outcomes. The aim of this study was to assess the effects of tea, theanine alone, or theanine plus caffeine on cognition, mood, and sleep outcomes, using data from randomized controlled trials (RCTs) in healthy participants. The Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, Embase, and Ovid Medline were searched up to and including August 2023. Data relevant to the participants, interventions, comparisons, outcomes (ie, cognition, mood, and sleep), and study design were extracted. Fifty RCTs were included in the review, of which 15 were eligible for at least 1 meta-analysis, most commonly performed by use of standardized mean differences (SMD), in random effects models. After intake of theanine plus caffeine, and of placebo, small-to-moderate differences were found between these interventions in the first hour (h1) and second hour (h2), that favoured theanine plus caffeine, with regard to cognition and mood outcomes such as choice reaction time (h1: SMD, -0.48; 95% CI, -1.01, 0.05), digit vigilance task accuracy (h2: SMD, 0.20; 95% CI, 0.02, 0.38), attention switching task accuracy (h2: SMD, 0.33; 95% CI, 0.13, 0.54), and overall mood (h2: SMD, 0.26; 95% CI,-0.10, 0.63). There was a small-to-moderate difference between the effects of theanine and placebo that favored theanine on choice reaction time (h1: SMD, -0.35; 95% CI,-0.61, -0.10). The CIs frequently highlighted the uncertainty surrounding the direction and magnitude of these differences. This meta-analysis provides evidence that theanine plus caffeine, and theanine alone, could be beneficial for cognitive and mood outcomes. More research using tea beverages or tea-equivalent bioactive doses and research in free-living participants is needed. CRD42022351601.