单一登录（英语：Single sign-on，缩写为 SSO），又译为单一签入，一种对于许多相互关连，但是又是各自独立的软件系统，提供访问控制的属性。当拥有这项属性时，当用户登录时，就可以取得所有系统的访问权限，不用对每个单一系统都逐一登录。这项功能通常是以轻型目录访问协议（LDAP）来实作，在服务器上会将用户信息保存到LDAP数据库中。相同的，单一注销（single sign-off）就是指，只需要单一的注销动作，就可以结束对于多个系统的访问权限。

Single sign-on (SSO) is a property of access control of multiple related, but independent software systems. With this property a user logs in with a single ID and password to gain access to a connected system or systems without using different usernames or passwords, or in some configurations seamlessly sign on at each system. This is typically accomplished using the Lightweight Directory Access Protocol (LDAP) and stored LDAP databases on servers also called directory servers. A simple version of single sign-on can be achieved over IP networks using cookies but only if the sites share a common DNS parent domain. For clarity it is best to refer to systems requiring authentication for each application but using the same credentials from a directory server as Directory Server Authentication and systems where a single authentication provides access to multiple applications by passing the authentication token seamlessly to configured applications as SSO.