When employees feel that the psychological contract has been breached, their turnover intention increases and their commitment to work decreases. These behaviors can be shaped by the support employees receive from their co-workers. Drawing on theoretical findings on turnover intention, psychological contract violation, work engagement, and co-worker support, this article explores the internal linkages and relationships between the key constructs, discussing the possible causes and consequences of the decision to quit. Data from a survey collected by CATI on 500 knowledge workers employed in the Business Services Sector in Poland was used. The analysis was conducted using the structural equation modeling method (SEM). The psychological contract violation is positively correlated with knowledge workers’ intention to leave, work engagement mediates the effect of psychological contract violation on intention to leave, and knowledge workers’ support modifies the effect of psychological contract violation on intention to leave. In particular, the results obtained have theoretical and practical relevance for enhancing organizational management theories and practices, including human resource management, encouraging knowledge workers to reach their full potential, lowering intentions to leave that have a negative impact on the organization, and modifying HR procedures that foster positive attitudes.