Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured from 1966, delivered from December 1967, and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, especially as the TOPS-10 operating system became widely used.

The PDP-10's architecture is almost identical to that of DEC's earlier PDP-6, sharing the same 36-bit word length and slightly extending the instruction set. The main difference was a greatly improved hardware implementation. Some aspects of the instruction set are unusual, most notably the byte instructions, which operate on bit fields of any size from 1 to 36 bits inclusive, according to the general definition of a byte as a contiguous sequence of a fixed number of bits.

The PDP-10 was found in many university computing facilities and research labs during the 1970s, the most notable being Harvard University's Aiken Computation Laboratory, MIT's AI Lab and Project MAC, Stanford's SAIL, Computer Center Corporation (CCC), ETH (ZIR), and Carnegie Mellon University. Its main operating systems, TOPS-10 and TENEX, and to a lesser extent ITS at a small number of influential locations, were used to build out the early ARPANET. The 36-bit words were especially well suited to Lisp at the time, because split in two parts 18 bits each to be used as pointers, it could address 256K objects instead of 64K on an equivalent 32-bit machine. For these reasons, the PDP-10 looms large in early hacker folklore.

Projects to extend the PDP-10 line were eclipsed by the success of DEC's unrelated VAX superminicomputer, and the cancellation of the PDP-10 line was announced in 1983. According to reports, DEC sold "about 1500 DECsystem-10s by the end of 1980".