At the end of high school, teenagers must deal with the first life-changing decision of determining what to do after graduation. For these decisions, adolescents need to be able to make good choices. However, most schools have not yet implemented decision trainings into their curricula. A new intervention called "KLUGentscheiden!" was developed to train complex decision-making in high school students to close this gap. The intervention targets three key components of good decision-making: envisioning one's objectives, identifying relevant alternatives, and comparing the identified alternatives by a weighted evaluation. We assumed that successfully training those decision-analytical steps should enhance self-perceived proactive decision-making skills. In addition, the training should also enhance self-assessed career choice self-efficacy. The intervention was evaluated in a pseudorandomized control study including 193 high school students. Compared to a control group, the intervention group significantly increased proactive decision-making skills and career choice self-efficacy. Although different long-term evaluations are still pending, the "KLUGentscheiden!" intervention provides an important tool to train complex decision-making in high-school students. It also has the potential to apply to other career choices of young individuals, such as choosing majors, a final thesis, a job, or a field of work.