Data attribution in text-to-image generative models is a crucial yet underexplored problem, particularly at the regional level, where identifying the most influential training regions for generated content can enhance transparency, copyright protection, and error diagnosis. Existing data attribution methods either operate at the whole-image level or lack scalability for large-scale generative models. In this work, we propose a novel framework for region-level data attribution. At its core is the Attribution Region (AR) detector, which localizes influential regions in training images used by the text-to-image generative model. To support this research, we construct a large-scale synthetic dataset with ground-truth region-level attributions, enabling both training and evaluation of our method. Empirical results show that our method outperforms existing attribution techniques in accurately tracing generated content back to training data. Additionally, we demonstrate practical applications, including identifying artifacts in generated images and suggesting improved replacements for generated content. Our dataset and framework will be released to advance further research in region-level data attribution for generative models.