Forty-four million U.S. workers hold a flexible work role in the “gig economy” in conjunction with a traditional work role. This supplementary work role is known as a side-hustle, or income-generating work performed on the side of a full-time job. Whereas organizations and scholars have tended to view side-hustles as an activity that diminishes employee performance, employees may enjoy benefits from side-hustles. Indeed, research points to the benefits of accumulating multiple roles outside of work (e.g., volunteering or family roles). I investigate these disparate perspectives about the positive and negative implications of a SHR for performance in full-time work. To do so, I draw on boundary theory, which suggests that the degree of similarity between two roles, whether different from one another or blurring together, shapes how roles affect attitudes and behavior. I tested my predictions about how SHRs influence full-time work performance in a four-wave field study of 276 employees and 170 supervisors. Specifically, I address similarity between a SHR and FWR (SHR-FWR similarity), or the number of similar requirements between a SHR and FWR and extent of those similarities. I argue that SHR-FWR similarity has a negative relationship with boundary negotiation efforts because transitions between similar roles require little psychological effort. This relationship was not supported by my findings. I also assert that SHR-FWR similarity decreases psychological detachment from full-time work as similar roles blur together and limit recovery from full-time work. This relationship was supported by my findings. I further argue that side-hustle meaningfulness moderates the relationship between SHR-FWR similarity and boundary negotiation efforts and psychological detachment from full-time work. This prediction was supported for the effect on psychological detachment from full-time work. Finally, I examined how the effects of SHR-FWR similarity carry through to full-time work performance via exhaustion. These indirect effects were not supported. A supplemental polynomial regression analysis in which I examined status consistency was more fruitful. I found that status inconsistencies between a SHR and FWR led to increased role stress within full-time work. I conclude with a discussion of alternative approaches to understanding the confluence of SHRs and FWRs and practical implications.