功能需求（functional requirement）为一软件工程用语，功能需求定义一个软件系统或组件的功能，也是一个系统需提供的功能及服务。功能可以用一组输入、行为及输出的组合来表示。功能需求可以是计算、技术细节、数据处理或其他说明系统希望达成功能的内容。功能需求会以非功能性需求（或是品质需求）为其基础，后者会描述设计或实现时的限制条件（例如性能需求、保安性或可靠度等）。

若依照需求工程的定义，功能需求会列出系统的特定结果，而非功能性需求会列出系统的一些整体特性（如成本及可靠度），功能需求有关系统的应用架构，而非功能性需求和系统的技术架构有关。

In product development and process optimization, a requirement is a singular documented physical and functional need that a particular design, product or process must be able to perform. It is most commonly used in a formal sense in systems engineering, software engineering, or enterprise engineering. It is a statement that identifies a necessary attribute, capability, characteristic, or quality of a system for it to have value and utility to a customer, organization, internal user, or other stakeholder. A requirement specification (often imprecisely referred to as the spec, because there are different sorts of specifications) refers to an explicit set of requirements to be satisfied by a material, design, product, or service.