1. Early-life stressful conditions can shape individual phenotypes and ultimately influence fitness. Oxidative stress is a pervasive threat that affects many fitness-related traits and can modulate life-history trade-offs. Yet, the extent to which exposure to oxidative stress during early life can have long-lasting effects on key fitness-related traits remains to be elucidated, particularly in natural populations of vertebrates. 2. Using a wild population of great tits Parus major, we experimentally dosed 11 day-old birds with paraquat, a pro-oxidant molecule, aiming at increasing oxidative stress. One year later, we recaptured 39 of them as adult recruiting breeders and quantified effects of the paraquat exposure on their resistance to oxidative stress, carotenoid-based plumage colouration and male sperm performance. 3. Despite the absence of a short-term effect of paraquat on oxidative stress measured two days later, the pre-fledging exposure to paraquat induced a reduction in individual oxidative damage measured at adulthood. Paraquat-dosed individuals also had brighter plumage, but no effect was observed on male sperm performance. 4. For the first time in a natural population of vertebrates, we experimentally show that an early-life acute exposure to a pro-oxidant has long-lasting effects on individual resistance to oxidative stress at adulthood. Our results are in line with the environmental matching and the hormesis hypotheses but may also reflect selective disappearance of individuals with lower resistance to oxidative stress.