This book constitutes the proceedings of the First MICCAI 2023 Challenge on Semi-supervised Tooth Segmentation, SemiToothSeg 2023, held in Conjunction with MICCAI 2023, in Vancouver, BC, Canada, on October 8, 2023. The 16 full papers presented in this book were carefully reviewed and selected from 64 submissions. The papers were written by participants in the STS challenge to describe their solutions for automatic teeth segmentation using the offcial training dataset released for this purpose. In general, this challenge aims to promote the development of the teeth segmentation in panoramic X-ray images and dental CBCT scans.