YMODEM is a file transfer protocol used between microcomputers connected together using modems. It was primarily used to transfer files to and from bulletin board systems. YMODEM was developed by Chuck Forsberg as an expansion of XMODEM and was first implemented in his CP/M YAM program. Initially also known as YAM, it was formally given the name "YMODEM" in 1985 by Ward Christensen, author of the original XMODEM.

YMODEM extended XMODEM in three ways, combining features found in other extended XMODEM varieties. Like XMODEM-CRC, YMODEM replaced the 8-bit checksum with a 16-bit cyclic redundancy check (CRC), but made it the default form of correction instead of optional. From TeLink it added the "block 0" header that sent the filename and size, which allowed batch transfers (multiple files in a single session) and eliminated the need to add padding at the end of the file. Finally, YMODEM allowed the block size to be increased from the original 128 bytes of data to 1024, as in XMODEM-1k, which greatly improved throughput on faster modems.

Forsberg built the standard with all of these features as runtime options, allowing a single protocol driver to fall back to XMODEM-CRC or even XMODEM when connecting to non-YAM systems. He believed that programmers would want to implement as many of these features as possible on any given platform. He was dismayed to find that the majority of implementations were actually providing nothing more than 1k block size with CRC-16, failing to implement the "block 0" while continuing to use the YMODEM name. The result was the release of many mutually incompatible YMODEM implementations, and the use of the name YMODEM Batch to clearly indicate those versions that did support the complete standard.