Harm-minimization strategies aim to reduce gambling-related risks; however, minimal evidence supports the effectiveness of current strategies involving the placement of warning signs in gambling venues and on electronic gaming machines (EGMs). This qualitative replication study evaluated the differential effect of pop-up messages compared to static signs and the content of messages on EGMs on recall, thoughts, and behaviors assessed during the session and at 2-week follow-up. In Study 1, 127 regular EGM gamblers (male = 97, mean age = 20.3) recruited from a university student population attended a laboratory where they were randomly assigned to play a computer-based simulated EGM analogue displaying signs that differed by (a) mode of presentation (pop-up and static) and (b) message content (informative, self-appraisal, and control/blank). In Study 2, an identical methodology was used but included the use of a simulated EGM within an in vivo gaming setting with 124 regular EGM players (male = 81, mean age = 44.1). Results from both studies showed that pop-up messages were recalled more effectively than static messages immediately and at 2-week follow-up. Pop-up messages reportedly had a significantly greater impact on within-session thoughts and behaviors. Messages encouraging self-appraisal resulted in significantly greater effect on self-reported thoughts and behaviors during both the experimental session and in subsequent EGM play. These findings support the effectiveness of pop-up messages containing self-appraisal messages as an appropriate harm-minimization initiative.