The use of inquiry-based learning (IBL) is encouraged in schools, as it has been shown to be an effective method for raising students' motivation in STEM subjects and increasing their understanding of scientific concepts. Nevertheless, IBL is not very often used in classrooms by teachers due to different (perceived) obstacles. Within the Ark of Inquiry project, teacher training sessions were designed that enabled the teachers to experience IBL from different perspectives: Teacher as a learner, teacher as a thinker, and teacher as a reflective practitioner. We expected that the trainings would have an impact on teachers' sense of efficacy (TE), which has been shown to be positively related to teachers' readiness to adopt new teaching methods, and their attitudes toward IBL. Four hundred and ninety-seven teachers from 10 countries were involved in the study. We found that teachers' higher sense of efficacy was related to more positive attitudes toward IBL before the training. The teacher training sessions had a positive effect on the Student Engagement Subscale of TE (d = 0.16) and attitudes toward IBL. The strongest positive effects on attitudes were related to the perceived available resources for teaching inquiry (d = 0.36) and inquiry being suitable for motivating different students (d = 0.28). However, the training did not impact how teachers perceive systemic restrictions. The study concludes that this kind of teacher training can be a suitable method of boosting TE and overcoming some perceived obstacles for adopting IBL in the classroom.