Adolescents' and parents' reactions to pubertal development are hypothesized to contribute to changes in family dynamics. Using 7‐year longitudinal data from the NICHD‐SECCYD (488 boys, 475 girls), we examined relations between pubertal development (timing, tempo) and trajectories (developmental change and year‐to‐year lability) of parent–child conflict and closeness from age 8.5 to 15.5 years. Changes were mostly characterized by year‐to‐year fluctuations—lability. Parent–child conflict increased and closeness decreased some with age. Pubertal timing and tempo were more consistently associated with lability in parent–child relationships than with long‐term trends, although faster tempo was associated with steeper decreases in parent–child closeness. Findings provide a platform for examining how puberty contributes to both long‐term and transient changes in adolescents' relationships and adjustment.