软件即服务（英语：Software as a Service，缩写为 SaaS，发音：sæs或sɑs）有时被作为“即需即用软件”（即“一经要求，即可使用”）提及，它是一种软件交付模式。在这种交付模式中云端集中式托管软件及其相关的数据，软件仅需透过互联网，而不须透过安装即可使用。用户通常使用精简客户端经由一个网页浏览器来访问软件即服务。

对于许多商业应用来说，软件即服务已经成为一种常见的交付模式。这些商业应用包括会计系统、协同软件、客户关系管理、管理信息系统、企业资源计划、开票系统、人力资源管理、内容管理、以及服务台管理。软件即服务已经被吸纳进所有领先的企业级软件公司的战略中。这些公司的最大的卖点之一就是通过将硬件和软件维护及支持外包给软件即服务的提供者，来降低信息技术（Information Technology，简称IT）成本。

Software as a service (SaaS; pronounced /sæs/) is a software licensing and delivery model in which software is licensed on a subscription basis and is centrally hosted. It is sometimes referred to as "on-demand software". SaaS is typically accessed by users using a thin client via a web browser. SaaS has become a common delivery model for many business applications, including office and messaging software, payroll processing software, DBMS software, management software, CAD software, development software, gamification, virtualization, accounting, collaboration, customer relationship management (CRM), management information systems (MIS), enterprise resource planning (ERP), invoicing, human resource management (HRM), talent acquisition, content management (CM), antivirus software, and service desk management. SaaS has been incorporated into the strategy of all leading enterprise software companies."IBM cloud strategy".  One of the biggest selling points for these companies is the potential to reduce IT support costs by outsourcing hardware and software maintenance and support to the SaaS provider.