Antibody-drug conjugates (ADCs) have emerged as a transformative modality in the treatment of solid tumors. YL201, a novel B7H3-targeting ADC, leverages a tumor microenvironment activable linker-payload platform, coupled with a novel topoisomerase 1 inhibitor via a protease-cleavable linker. Here we report the findings from a large-scale, global, multicenter, phase 1 trial evaluating the safety, pharmacokinetics and preliminary efficacy of YL201 in patients with advanced solid tumors refractory to standard therapies. The trial included a dose-escalation part (phase 1) and a dose-expansion part (phase 1b). A total of 312 patients were enrolled across multiple tumor types, including extensive-stage small cell lung cancer (ES-SCLC), nasopharyngeal carcinoma (NPC), non-small cell lung cancer, esophageal squamous cell carcinoma and other solid tumors. The maximum tolerated dose was determined to be 2.8 mg kg-1, and the recommended expansion dose was selected as 2.0 mg kg-1 and 2.4 mg kg-1 every 3 weeks. The most common grade 3 or higher treatment-related adverse events included neutropenia (31.7%), leukopenia (29.5%) and anemia (25.0%). Only 4 cases of interstitial lung disease (1.3%) and 1 case of infusion reactions (0.3%) were observed. Encouraging anti-tumor activity was observed, particularly in patients with ES-SCLC (objective response rate (ORR), 63.9%), NPC (ORR, 48.6%), lung adenocarcinoma (ORR, 28.6%) and lymphoepithelioma-like carcinoma (ORR, 54.2%). No significant correlation between B7H3 membrane expression and the ORR was found. YL201 demonstrated an acceptable safety profile and a promising efficacy in heavily pretreated patients with advanced solid tumors, particularly in those with ES-SCLC, NPC or lymphoepithelioma-like carcinoma. Phase 3 clinical trials for patients with SCLC and NPC have already been initiated. ClinicalTrials.gov identifiers: NCT05434234 and NCT06057922.