This longitudinal study examines how academic psychological capital mediates between parent-child relationships and academic performance in a group of high school students. The sample consisted of 402 students (217 girls and 187 boys) aged between 12 and 17 years. Using a three-wave design, as hypothesized, a significant indirect effect was found between (good) parent-child relationships (assessed at time 1) and academic performance (assessed at time 3) via academic PsyCap (assessed at time 2). Students who perceived high-quality relationships with their parents reported high levels of academic psychological capital and obtained better objective academic performance over time. Theoretical and practical implications of the results are discussed, as well as strengths and weaknesses and future research directions.