Background: Friendships have been linked to mental health and school attainment in children. The effects of friendlessness and friendship quality have been well researched, but less is known about the role of friendship stability (i.e., maintaining the same friend over time), an aspect of friendship which is often interrupted by the transition between phases of schooling. Many children report concerns about the secondary school transition which introduces a number of new social and academic challenges for children. Aims: To explore rates of friendship stability and whether maintaining a stable best friend across the primary to secondary school transition provided benefits to children's adjustment during this period. Sample: Data were from 593 children (M age = 11 years 2 months). Methods: This study used longitudinal data from children transitioning into 10 UK secondary schools and explored the association between self-reported friendship stability and three outcomes: academic attainment, emotional problems and conduct problems. Analyses controlled for friendship quality and pre-transition psychological adjustment or attainment as appropriate. Results: Rates of friendship stability were relatively low during this period. Children who kept the same best friend had higher academic attainment and lower levels of conduct problems. Exploratory analyses indicated that secondary school policies that group children based on friendships may support friendship stability. Conclusions: Helping maintain children's best friendships during the transition to secondary school may contribute to higher academic performance and better mental health.