There is an urgent need to understand the implementation barriers of antimicrobial stewardship programs (ASP) in low- and middle-income countries. We conducted a mixed-methods study in public and private hospitals across all provinces in Indonesia (March-December 2023). We used a self-assessment questionnaire with a scoring system, and multilevel ordinal regression to assess associations with hospital and district-level characteristics. Focus group discussions (FGD) with hospital stakeholders examined barriers and enablers. We applied a patient safety framework to integrate results. 575 (19%) of 3026 hospitals completed the self-assessment, of whom 516 (89.7%) had a formal ASP (median 4 [IQR1-5] years), and 14 participated in FGD. The median overall ASP development score was 48.4% (35.9-62.5%), classifying 41 (8.0%) hospitals as inadequate (0-25%), 237 (45.9%) as basic (26-50%), 179 (34.7%) as intermediate (51-75%) and 59 (11.4%) as advanced (76-100%). Scores were highest for hospital leadership support (83.4% [66.7-100%]), followed by ASP team and infectious disease training (66.7% [55.6-77.8%]); education (50% [0.0-75.0%]); ASP interventions (43.8% [18.7-68.7%]); hospital infrastructure (42.9% [14.3-71.4%]); and monitoring, reporting and feedback (40.9% [27.3-54.5%]). A higher ASP development score was associated with higher hospital tiered level, longer ASP duration, and higher district-level public health development index and per capita domestic expenditure, but not with hospital ownership or geographic region. FGDs highlighted barriers related to hospital leadership support, staff technical and behavioral skills, cross-disciplinary collaboration, fear of loss of prescriber autonomy, microbiology and IT support, and hospital accreditation. Identified implementation barriers can inform actions for context-specific, sustainable improvement of ASPs.