The widespread adoption of distributed systems and microservices architectures has significantly increased the reliance on Application Programming Interfaces (APIs) for communication between services, applications, and external platforms. While APIs enable seamless integration and scalability, they also introduce critical security vulnerabilities that can be exploited by malicious actors. This study presents a comprehensive analysis of API security in distributed systems, focusing on common threats, security frameworks, and best practices for protecting APIs in complex, interconnected environments. he paper examines key API security challenges, including authentication and authorization weaknesses, data exposure, injection attacks, rate limiting issues, and misconfigurations. It explores widely adopted security mechanisms such as OAuth 2.0, OpenID Connect, API gateways, and token-based authentication, along with encryption protocols like TLS to ensure secure communication. The role of API management platforms and service meshes in enforcing security policies and monitoring API traffic is also discussed. Furthermore, the study highlights the importance of integrating security into the software development lifecycle through DevSecOps practices, enabling continuous testing, vulnerability assessment, and automated threat detection. Emerging technologies such as artificial intelligence and machine learning are examined for their potential to enhance API security through anomaly detection and predictive threat analysis. The findings emphasize that a layered, proactive, and policy-driven approach is essential to securing APIs in distributed systems, ensuring data integrity, confidentiality, and system resilience.\\n\\n