YMODEM is a protocol that is used between two computers to transfer files. YMODEM was developed by Chuck Forsberg and is the successor to the XMODEM protocol. In contrast to the XMODEM protocol that uses 128- byte blocks for data transmission, the block size was increased to 1 KiB, and also the file name is in the first block of data transmitted. YMODEM was further developed to ZMODEM.

A variant Enhanced YMODEM was developed in 1994 by Herwig Feichtinger at Shamrock Software. It avoids gaps in the transfer by each successive block without the check digit (CRC) will be sent without confirmation in advance, while the check digits are transmitted only after receipt of the confirmation of the previous block. Since no character escaping takes place, the achievable transfer rate is slightly higher than ZMODEM.