A port memory, also known as memory-mapped I/O (Input and Output), is, along with port-mapped I/O, a means to connect external devices to a computer's central processing unit.

In memory-mapped I/O, each device has a separate address that is mapped into the system memory, along with random-access memory (RAM) and read-only memory (ROM). The machine's normal memory access instructions enables a device to be activated and used.

An advantage of memory-mapped I/O is that every instruction that can convey information to memory can be used to manipulate an I/O device, such as a printer or a monitor. A disadvantage to this mode of operation is that the entire identifying address must be fully decoded for every device. For extensive addresses, this can add to hardware costs.