Climate change poses a concern globally, as it adversely affects planetary and population health. Extreme heat is one of the largest weather-related causes of mortality, projected to cause severe heatwaves and droughts globally. Current research has focused on the effects of outdoor heat exposure on population health risks, leaving a gap in knowledge regarding indoor exposure to extreme heat. This paper presents evidence for the effects of extreme heat, both indoors and outdoors, on non-communicable diseases (NCDs) in the United Kingdom. This study applies a systematic review methodology to identify, quality appraise, extract, and summarize findings from studies reporting on the associations between extreme heat and population health risk of NCDs. The literature search was conducted across six electronic databases. `There were 244 studies identified. Twenty-three studies met the inclusion criteria, of these 16 studies met the quality benchmark. Fourteen studies examined the links between heat exposure and mortality, while the remaining studies focused on emergency hospital admissions and years of life lost. The review highlights a consistent association between exposure to extreme heat and increased risk of mortality. Further research is needed to explore the effects of indoor extreme heat on the incidence of NCDs and related outcomes.