Online courses have become an increasingly attractive format for delivering teacher training. However, the low retention rates are a critical and still unsolved issue. This paper presents the results of a randomized controlled trial aimed at testing the impact of a personalized support model on teachers' retention in online training courses. The support consisted of a package of nine messages triggered by teachers' characteristics and their specific (in)actions on the course platform. The study involved 3,777 lower-secondary education professional and student teachers from nine European Union Member States and Turkey, who were invited to participate in four new online courses in school year 2018/2019. The experimental estimates show that the offered support increased course completion by 10 percentage points among professional teachers in EU Member States, while it had no effects among student teachers nor in Turkey. Implications for online teacher training providers--such as the importance of reaching out to participants with poor online training experience and who do not start the courses in time--are discussed.