Portals is a low-level network API for high-performance networking on high-performance computing systems developed by Sandia National Laboratories and the University of New Mexico.

Portals is based on the concept of elementary building blocks that can be combined to support a wide variety of upper-level network transport semantics. Portals provides one-sided data movement operations, but unlike other one-sided programming interfaces, the target of a remote operation is not a virtual address. Instead, the ultimate destination in memory of an incoming message is determined at the receiver by comparing contents of the message header with the contents of structures at the destination. This flexibility allows for efficient implementations of both one-sided and two-sided communications. In particular, Portals is aimed at providing the fundamental operations necessary to support a high-performance and scalable implementation of the Message Passing Interface (MPI) standard. It was also used as the initial network transport layer for the Lustre file system.

The Portals 4 specification is still in draft form and the specification is seeing small changes on a regular basis. The latest draft is available on the Specification wiki page.