Setting up cross-account access between AWS Lambda and S3 is a common requirement in enterprise environments where resources are distributed across multiple AWS accounts for security, compliance, or organizational reasons. This guide provides a comprehensive walkthrough of establishing secure cross-account access, covering IAM role configuration, bucket policies, and practical implementation patterns.

Cross-account access enables Lambda functions in one AWS account (Account A) to securely access S3 buckets in another account (Account B). This pattern is essential for data processing workflows, backup operations, and multi-account architectures where centralized data storage serves multiple application accounts.