Parent and child endorsement of reactive aggression both predict the emergence of child aggression, but they are rarely studied together and in longitudinal contexts. The present study does so by examining the unique predictive effects of parent and child endorsement of reactive aggression at age 8 on child aggression at age 9 in 1456 children from 13 cultural groups in 9 nations. Multiple group structural equation models explored whether age 8 child and parent endorsement of reactive aggression predicted subsequent age 9 child endorsement of reactive aggression and child aggression, after accounting for prior child aggression and parent education. Results revealed that greater parent endorsement of reactive aggression at age 8 predicted greater child endorsement of aggression at age 9, that greater parent endorsement of reactive aggression at age 8 uniquely predicted greater aggression at age 9 in girls, and that greater child endorsement of reactive aggression at age 8 uniquely predicted greater aggression at age 9 in boys. All three of these associations emerged across cultures. Implications of, and explanations for, study findings are discussed.