Identifying oneself and being identified by others as a leader (vs. a follower) is a critical aspect of informal leadership. But what happens when an organizational member's personal leader identity differs from how others identify them? Grounded in stress appraisal theory, this study explores the individual-level implications of (in)congruence between self- and other-identification as a leader or follower. We develop a conceptual model that explains how different forms of leader identity (in)congruence generate stress appraisals that influence the focal individual's in-role performance. We then describe two complementary studies testing the model. Study 1 is a multiwave, multisource field study of 226 coworker dyads. Study 2 is a controlled experiment with 648 full-time employees that assesses the causal relationship between different forms of leader identity (in)congruence and stress appraisals, as well as the generalizability of our findings to other-identification by an entire team. Across both studies, we find that identity incongruence (particularly when the focal individual identifies as a leader but others identify them as a follower) prompts hindrance stress appraisals that reduce in-role performance. In contrast, identity congruence (particularly congruence in identification as a leader) encourages challenge stress appraisals that enhance in-role performance. (PsycInfo Database Record (c) 2023 APA, all rights reserved).