A large body of scientific evidence has established the impact of increased temperatures on human health. This study aimed to collate the research findings on the effects of extreme heat on direct heat illness (heat illnesses such as heat stroke or heat exhaustion, or dehydration) in a systematic review and meta-analysis and to provide evidence for needed public health intervention. We conducted a search of peer-reviewed articles in three electronic databases, from database inception until January 2022. A random-effects meta-analysis model was used to calculate the pooled relative risks (RRs) of the association between high temperature and direct heat-related outcomes. A narrative synthesis was also performed for studies analysing heatwave effects. Assessment of evidence was performed in three parts: individual study risk of bias; quality of evidence across studies; and overall strength of evidence. A total of 62 studies meeting the eligibility criteria were included in the review, of which 29 were qualified to be included in the meta-analysis. The pooled results showed that for every 1°C increase in temperature, direct heat illness morbidity and mortality increased by 18% (RR 1.18, 95%CI: 1.16-1.19) and 35% (RR 1.35, 95%CI: 1.29-1.41), respectively. For morbidity, the greatest increase was for heat illness (RR 1.45, 95%CI: 1.38 -1.53), compared to dehydration (RR 1.02, 95%CI: 1.02-1.03). Heatwave studies also provided strong evidence of increase morbidity and mortality from direct heat illness during heatwaves. We conclude that increased temperature leads to higher burden of disease from direct heat illness. Preventative efforts should be made to reduce direct heat illness during hot weather, targeting on the most vulnerable populations. This is especially important in the context of climate change.