Job engagement denotes the extent to which an employee invests the full self in performing the job. Extant research has investigated the positive outcomes of job engagement, paying little attention to its potential costs to the organizations. Integrating the extended self theory and the literature on psychological ownership as our overarching theoretical framework, we develop and test the double-edged effects of job engagement on workplace outcomes through the mediating role of job-based psychological ownership. Analyses of two survey studies with multisource multiphase data support that job engagement can lead to positive workplace outcomes including in-role performance and organizational citizenship behaviors (OCBs) through job-based psychological ownership. At the same time, job engagement is also positively related to negative workplace outcomes including territorial behavior, knowledge hiding, and pro-job unethical behavior through the same mechanism of job-based psychological ownership. These indirect effects of job engagement on negative work outcomes are amplified by employees’ avoidance motivation. The theoretical and practical implications are discussed.(PsycInfo Database Record (c) 2020 APA, all rights reserved)