Exposure therapy is considered the leading evidence-based treatment for anxiety disorders. Yet, a significant number of individuals fail to benefit from its efficacy due to their reluctance to confront themselves with the feared situation. With evidence indicating a link between approach motivation and reward processing in socially anxious individuals, prospective mental imagery may serve to enhance anticipatory reward processes and promote approach motivation for exposure to feared situations and subsequently behavioral engagement with feared social situations. The present study tested the impact of prospective mental imagery on anticipatory reward processes, approach motivation and behavioral engagement. Socially anxious participants (N = 136) were randomly assigned to either a prospective mental imagery group or a psychoeducation-only control group. Anticipatory reward processes and approach motivation were assessed before and after the intervention and behavioral engagement one week later. The prospective mental imagery group reported greater increases in anticipated and anticipatory pleasure as well as approach motivation towards feared social situations compared to the control group. No differences in reward anticipation and behavioral engagement with feared social situation were found. Mediational analyses indicated that the association between prospective mental imagery and increases in approach motivation was statistically mediated by increases in anticipatory reward processing, although the lack of temporal precedence between mediator and outcome precludes causal conclusions. Exploratory analyses further suggest that higher vividness of mental imagery was associated with greater efficacy of prospective mental imagery on reward processing and approach motivation.