Delays in breast cancer (BC) diagnosis and treatment negatively impact survival outcomes. Understanding patient- and provider-related factors behind these delays is crucial. This study aimed to identify nationwide reasons for delayed diagnosis and treatment of locally advanced BC in Turkiye. A prospective, multicenter hospital-based survey was conducted across 35 institutions between 2023 and 2024. Patient- and provider-related delays were assessed via a structured 61-item face-to-face survey, supplemented by clinical data from electronic health records. Delays exceeding 3months were clinically categorized as significant. A total of 1322 women participated from seven regions across Turkiye. Factors contributing to diagnostic delays on a national level included economic reasons (5.5%), lack of family support (3.3%), lack of knowledge (12.4%), lack of time due to household work (3.8%), difficulty in finding an appointment (6.7%), pregnancy-related reasons (1.1%), fear of losing the breast (8.9%), fear of death (9.8%), and transportation difficulties (5.1%). Provider-related delays were infrequent. About 89.3% of the patients had the initial doctor appointment and 89.6% had the first specialist consultation within one month. Treatment planning was predominantly based on a multidisciplinary team decision in 88.3% of patients. Regarding treatment initiation, 93.2% started required treatment within 1 month of decision. Patient-related factors are the major causes of diagnostic delay in Turkiye. On the other hand, from the provider’s perspective, the presence of multidisciplinary teams, including dedicated breast surgeons, represents a key factor in ensuring the timely implementation of diagnostic procedures and treatment strategies.