Background: The burden of non-small cell lung cancer (NSCLC) remains high in Spain, with lung cancer accounting for 20% of cancer-related deaths annually. Programs such as the Spanish Thoracic Tumour Registry (TTR) and the global I-O Optimise initiative have been developed to observe patients in clinical practice with the aim of improving outcomes. This analysis examined treatment patterns and survival in patients with stage III NSCLC from the TTR. These patients represent a heterogenous group with complex treatment pathways. Methods: The TTR is an ongoing, observational, prospective, and retrospective cohort multicentre study (NCT02941458) that follows patients with thoracic cancer in Spain. Adults aged ≥18 years with stage IIIA/IIIB NSCLC enrolled in the TTR between 01 Jan 2010 and 31 Oct 2019 were included in this analysis.Initial treatment received was described by cancer stage and histology (squamous and non-squamous NSCLC). Kaplan-Meier estimates of progression-free survival (PFS) and overall survival (OS) were calculated over a 5-year period. Results: A total of 1,838 patients were included in the cohort, including 1,082 with stage IIIA (58.9%) and 756 with stage IIIB (41.1%). Median follow-up was 18.3 months. The median age of patients was 66 years, and most had non-squamous NSCLC (54.0%), were male (81.2%), and were active or former smokers (93.4%). Overall, 26.3% of patients received surgical resection (37.0% for stage IIIA and 11.1% for stage IIIB). The most frequent initial treatment received was conc urrent chemoradiotherapy for stage IIIA (30.2%) and stage IIIB (37.0%) patients. Median OS was lower in patients with stage IIIB than stage IIIA (28 vs. 37 months) disease and was lower for patients with squamous than non-squamous histology (19 vs.26 months). Median PFS and OS varied when patients were stratified by initial treatment. Conclusions: This TTR analysis describes the clinical reality surrounding the initial management and survival outcomes for stage III NSCLC in Spain and presents survival outcomes comparable with other real-world evidence. It provides insights into the diverse approaches used before the availability of immunotherapies and targeted treatments in the non-metastatic NSCLC setting.