Background Suicidal thoughts and behaviors are major public health concerns in youth. Unfortunately, knowledge of reliable predictors of suicide risk in adolescents is limited. Promising research using a death stimuli version of the Implicit Association Test (Death IAT) indicates that stronger identification with death differs between adults with and without a history of suicidal thoughts and behaviors and uniquely predicts suicide ideation and behavior. However, research in adolescents is lacking and existing findings have been mixed. This study extends previous research by testing whether implicit identification with death predicts changes in suicide ideation during psychiatric treatment in adolescents. Methods Participants included 276 adolescents, ages 13–19, admitted to a short‐term residential treatment program. At hospital admission and discharge, adolescents completed the Death IAT and measures of recent suicidal thoughts. Results At admission, implicit identification with death was associated with recent suicide ideation, but did not differ between those who engaged in prior suicidal behavior and those who did not. Prospectively, adolescents' implicit identification with death at admission significantly predicted their suicide ideation severity at discharge, above and beyond explicit suicide ideation. However, this effect only was significant for adolescents with longer treatment stays (i.e., more than 13 days). Conclusions Implicit identification with death predicts suicidal thinking among adolescents in psychiatric treatment. Findings clarify over what period of time implicit cognition about death may predict suicide risk in adolescents.