Abstract: With the advent of the gig economy, profound changes have occurred in employee's work attitudes and practices, leading to the growing prevalence of side hustles. Drawing on Self-Determination Theory, this article explores how side hustles influence employees’ full-time job performance and further analyzes the mediating roles of self-challenge, multitasking, and social capital, as well as the moderating role of full-time work workload. The results indicate that employees' engagement in side hustles positively promotes self-challenge, multitasking, and social capital. Self-challenge and social capital mediate the impact of side hustles on full-time job performance. Additionally, full-time job workload positively moderates the enhancing effects of multitasking and social capital on full-time job performance. Furthermore, full-time job workload positively moderates the indirect effect of side hustles on improving full-time job performance through social capital. Consequently, corporate managers should fully explore the positive effects of side hustles on their primary business and support employees' career diversification.