Transactive memory is a system for encoding, storing, and retrieving information between people, where each person has knowledge of the other’s memory. Through two studies, we assessed whether transactive memory occurs in best friendships (N = 682). Results showed that transactive memory systems (TMSs) do exist in best friendships. Importantly, stronger TMSs are associated with higher friendship quality (satisfaction and commitment), and their strength is related to different friendship characteristics (e.g., trust). A novel method for assessing TMS structure was developed. Mixed-gender friendships were associated with more differentiated structures (different knowledge), and friendships higher in inclusion of other in the self were associated with more integrated structures (similar knowledge). These studies have implications for the quality and operation of friendships.