Background Approximately 20% of post‐institutionalized (PI) children exhibit disinhibited social engagement (DSE) or the propensity to approach and engage strangers. There is little longitudinal research examining changes in DSE after adoption, or methods of identifying children with persistent behaviors. Methods DSE was assessed observationally four times during the first 2 years postadoption in PI children 16–36 months at adoption (n = 68) relative to same‐age nonadopted children (n = 52). At age 5, a validated interview determined which PI children met criteria for Disinhibited Social Engagement Disorder (DSED). Results DSE trajectories initially increased and then stabilized. PIs had higher DSE levels initially and a steeper increase rate than NAs. When separated into physical and nonphysical DSE components, group differences arose in initial physical DSE and the rate of change of nonphysical DSE. DSE rate of increase predicted DSED diagnosis, as did longer institutional duration and poorer institutional care. Conclusions The rate of increase in DSE postadoption, rather than the level observed at adoption, is predictive of disordered social engagement by age 5 years.