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.