In our data-driven society, it is essential for students to become statistically literate. A core domain within Statistical Literacy is Statistical Inference, the ability to draw inferences from sample data. Acquiring and applying inferences is difficult for students and, therefore, usually not included in the pre-10th-grade curriculum. However, recent studies suggest that developing a good understanding of key statistical concepts at an early age facilitates the understanding of Statistical Inference later on. This study evaluates the effects of a Learning Trajectory for Statistical Inference on Dutch 9th-grade students' Statistical Literacy. Theories on informal Statistical Inference and repeated sampling guided the Learning Trajectory's design. For the evaluation, we used a pre-post research design with an intervention group (n = 267). The results indicated that students made significant progress on Statistical Literacy and on the ability to make inferences in particular, but also on the other domains of Statistical Literacy. To further interpret the learning gains of this group, we compared students' results with national baseline achievements from a comparison group (n = 217) who followed the regular 9th-grade curriculum, and with international studies using similar test items. Both comparisons confirmed a significant positive effect on all domains of Statistical Literacy. These findings suggest that current statistics curricula for grades 7-9, usually with a strong descriptive focus, can be enriched with an inferential focus.