The consolidation of three state-owned Islamic banks into PT Bank Syariah Indonesia (BSI) in 2021 represents a major strategic maneuver that yielded significant impact, yet conversely generated serious challenges regarding organizational culture harmonization and internal communication patterns. This study primarily focuses on exploring the internal communication methods deployed by PT Bank Syariah Indonesia to synergize its post-merger corporate culture, tracking manifestations of employee resistance, and examining management's mitigation steps to bridge cultural disparities, specifically within the Customer Care Group. Utilizing a descriptive qualitative approach, data collection was conducted through in-depth interviews, field observations, and document analysis concerning the implementation of the "BSI One Culture" corporate roadmap, which is guided by the AKHLAK core values. Empirical evidence confirms that internal communication strategies play a vital role in fostering employee loyalty and aligning perspectives amidst organizational heterogeneity. Although resistance emerged due to contrasting operational norms from each legacy bank, management utilized various communication channels, including massive socialization, staff engagement programs, and regular culture alignment forums. The conclusion highlights that successful cultural integration is heavily driven by the synchronization of strategic messaging and how employees respond to the communication stimuli provided. This study offers practical recommendations for BSI management to optimize employee engagement while providing a theoretical contribution to the evolution of organizational communication models within the landscape of Islamic financial institution mergers. Keywords: Internal Communication, Merger, Corporate Culture, BSI One Culture, Customer Care Group.