This study examined how college students’ levels of planned happenstance skills influenced the relationships among career engagement, career decision self-efficacy, and career decision certainty. Moderated mediation analysis was used with a sample of 217 Korean undergraduate students. The results indicated that career decision self-efficacy mediated the relationship between career engagement and career decision certainty. Moreover, the positive indirect effect of career engagement on career decision through career decision self-efficacy was strengthened as the level of planned happenstance skills increased. In conclusion, college students’ career engagement strengthens their career decision certainty via career decision self-efficacy when they have enough planned happenstance skills to discover unexpected career opportunities.