African conservation areas are internationally sought out as destinations to observe charismatic megafauna. Recently, research has identified that wildlife can become stressed at the presence of human observers and tourists. We investigated the impact of tourist presence and absence on the reintroduced lion Panthera leo Linnaeus, 1758 population in Addo Elephant National Park, South Africa, by measuring the frequency of disturbance-indicating (yawning, sitting, standing, moving away) and relaxation-indicating (rolling, grooming) behaviours when tourists were present and absent. Lions were significantly more likely to exhibit disturbance behaviours when tourists were present, and significantly more likely to perform relaxation behaviours when tourists were absent. We also measured the number of breaths per minute, as an indicator of stress, and found that this also increased in the presence of tourists. Lions incur stress and an energetic cost (albeit minor) from being observed by tourists. Some lion populations may face this chronically, which may increase their susceptibility to disease by reducing their immunity. Enforcing rules banning off-road driving in pursuit of wildlife and ensuring adequate refuge away from tourist infrastructure are important methods to minimise the stressful impacts of tourists on wildlife.