This study examines the associations of deep-level and surface-level similarity with team functioning (i.e., team effectiveness, team in-role performance, and team satisfaction). Hypotheses were tested using data from 420 teachers in 57 teams in secondary education. These teams were tightly integrated units in which teachers from different functional areas had to work together. From the results of multilevel analyses, it appeared that especially deep-level similarity was important for team functioning. Cross-level interactions showed a moderating effect of group social capital on the association of deep-level similarity with team functioning, but there was no such relationship found for surface-level similarity. Especially in teams with high group social capital, deep-level similarity was associated with improved team functioning.