Excessive warming from climate change has increased the total wildfire burned area over the past several decades in California. This has increased population exposure to both hazardous concentrations of air pollutants from fires such as fine particulate matter (smoke PM2.5) and extreme heat events. Exposure to PM2.5 and extreme heat are individually associated with negative health impacts and recent epidemiological evidence points to synergistic effects from concurrent exposures. This study characterizes the frequency and spatial distribution of co-occurring extreme heat and smoke PM2.5 events in California during the record-setting wildfire season of 2020. We measure exceedances over extreme thresholds of modeled surface-level smoke PM2.5 concentrations and heat index based on observed temperature and humidity. We estimate that, during the studied period, extreme smoke and heat co-occurred at least once within 68% of the State's area (~288,000 km2) and an average 2.5 times across all affected areas. Additionally, 16.5 million people, mostly in lower population density areas, were impacted at least once in 2020 by such synergistic events. Our findings suggest that public health guidance and adaptation policies should account for co-exposures, not only distinct exposures, when confronting heat and smoke PM2.5.