Climate change poses a major threat to the future of today's youth. Globally, young people are at the forefront of climate change activism. Their ability to engage, however, depends on the level of knowledge of climate change and concern about the topic. We sought to examine levels of knowledge and concerns about climate change among youth in South Africa, and their experiences of heat exposure. Ten questions on climate change knowledge, concerns and experiences were nested within a cross-sectional survey conducted in a cluster randomised trial among 924 secondary school learners in 14 public schools in low-income Western Cape areas. Learners' mean age was 15.8 years and they were predominately female. While 72.0% of respondents knew that climate change leads to higher temperatures, only 59.7% agreed that human activity is responsible for climate change, and 58.0% believed that climate change affects human health. Two thirds (68.7%) said that climate change is a serious issue and 65.9% indicated action is needed for prevention. Few learners indicated climate change events had affected them, although many reported difficulties concentrating during hot weather (72.9%). Female learners had lower knowledge levels than male learners, but more frequent heat-related symptoms. Learners scoring high on knowledge questions expressed the most concern about climate change and had the highest heat impacts. Many youth seem unaware that climate change threatens their future. Heat-related symptoms are common, likely undermining educational performance, especially as temperatures escalate. More is needed to mainstream climate change into South African school curricula.