Cloud computing has emerged as a cornerstone of modern computing infrastructure, offering unparalleled scalability and flexibility for diverse applications. However, the widespread adoption of cloud services has also raised significant concerns regarding security and scalability. As organizations increasingly rely on cloud environments to store and process sensitive data, ensuring robust security measures while maintaining scalability becomes paramount. This paper presents a comprehensive examination of secure and scalable architectures for cloud computing, addressing the challenges and proposing innovative solutions to enhance the resilience and efficiency of cloud infrastructures. The first section of the paper delves into the fundamental principles of cloud computing, outlining its key characteristics and architectural components. By understanding the underlying architecture of cloud systems, stakeholders can better appreciate the intricate interplay between security and scalability. Subsequently, the paper elucidates the multifaceted security threats facing cloud environments, ranging from data breaches and malicious attacks to insider threats and compliance issues. These challenges underscore the critical importance of implementing robust security mechanisms to safeguard sensitive data and infrastructure resources. In response to these challenges, the paper presents a systematic analysis of existing security mechanisms and architectural paradigms designed to fortify cloud environments. This includes encryption techniques, access control mechanisms, authentication protocols, and intrusion detection systems, among others. Furthermore, the paper explores the concept of defense-in-depth strategies, emphasizing the importance of layering security measures to mitigate the impact of potential breaches and vulnerabilities. In tandem with security considerations, the paper examines strategies for achieving scalability in cloud architectures. Scalability is a defining characteristic of cloud computing, enabling on-demand resource allocation and elastic scaling to accommodate fluctuating workloads. However, achieving scalability while ensuring security presents unique challenges, particularly in multi-tenant environments where resources are shared among disparate users. To address these challenges, the paper explores architectural frameworks such as microservices, containerization, and serverless computing, which offer inherent scalability benefits while facilitating robust isolation and security. Moreover, the paper investigates the role of automation and orchestration in optimizing cloud scalability and security. Through the use of DevOps practices and infrastructure- as-code tools, organizations can streamline deployment processes, enforce security policies, and dynamically scale resources in response to evolving demands. Additionally, the paper examines emerging technologies such as edge computing and federated architectures, which extend the scalability and security benefits of cloud computing to edge devices and distributed environments. In conclusion, this paper underscores the intrinsic link between security and scalability in cloud computing architectures. By adopting a holistic approach that integrates robust security measures with scalable design principles, organizations can mitigate risks, enhance operational efficiency, and unlock the full potential of cloud computing. As the landscape of cloud technologies continues to evolve, ongoing research and innovation in secure and scalable architectures will remain essential to address emerging threats and optimize performance in cloud environments.