The article considers the digital transformation of bank accounting through developing a comprehensive conceptual model of an integrated accounting system designed to support the functional architecture of a digital bank. The study elaborates theoretical foundations linking API-driven interoperability, cloud environments, big data frameworks and AI-supported automation with modern requirements for financial data integrity, continuous process synchronization and regulatory embeddedness, particularly in relation to IFRS-compliant measurement, recognition and disclosure procedures. The literature review identifies critical gaps including the absence of holistic integration architectures, fragmented coordination between accounting and risk-compliance subsystems, limitations in AI governance and control of algorithmic decisions, and insufficient empirical evidence on end-to-end accounting flows within open-banking environments. The research introduces a multilayered methodology encompassing design of unified data models, event-driven accounting mechanisms, embedded compliance and algorithmic verification modules, enabling the system to operate in real time with high levels of consistency, traceability and security. Moreover, a mathematical integration index is proposed to capture the multidimensional nature of system interoperability and to quantify its effects on accuracy, timeliness, reliability and interpretability of accounting outputs, enabling formal assessment of digital transformation outcomes. Empirical interpretations of the model indicate that the synergistic use of standardized APIs, scalable cloud platforms, advanced data-governance structures and AI-driven control mechanisms increases operational resilience, reduces data discrepancies and strengthens the capability of the bank to function as part of an open, networked fintech ecosystem. The proposed framework offers a theoretically grounded and practically oriented platform for the transition from isolated legacy accounting modules to fully integrated digital architectures capable of supporting continuous regulatory alignment, risk-informed decision-making and strategically oriented data-driven financial management.