NetBIOS (), or Network Basic Input/Output System, is an API that provides services related to the session layer of the OSI model, allowing applications on separate computers to communicate over a local area network. NetBIOS is not a networking protocol; NetBIOS API request and responses can be transported over several protocols.

The NetBIOS Frames protocol, which transports NetBIOS API requests and responses over IEEE 802.2 LLC, was commonly used as one of the layers of DOS and Microsoft Windows networking in the 1980s and 1990s. It was supported by a number of network operating systems, such as LAN Manager, LAN Server, Windows for Workgroups, Windows 95 and Windows NT.

The NetBIOS over IPX/SPX (NBX) protocol transports NetBIOS API requests and responses over Novell Netware's IPX/SPX protocols.

In modern networks, NetBIOS requests and responses are usually transported over TCP/IP via the NetBIOS over TCP/IP (NBT) protocol. NetBIOS over TCP/IP is also used for identifying system names in TCP/IP networks on Windows.