SITOR (SImplex Teletype Over Radio) is a system for transmitting text messages. It was developed in the 1960s by Koninklijke TNT Post as an improvement over radioteletype (RTTY). Although it uses the same frequency-shift keying (FSK) modulation used by regular RTTY, SITOR uses error detection, redundancy, and/or retransmission to improve reliability.

There are two SITOR modes:

SITOR sends 7-bit characters as a bit stream at 100 baud (which, in this case, is 100 bits per second, 10 milliseconds per bit, or 70 milliseconds per character).

The bitstream is FSK modulated with a 170 Hz frequency shift. The high frequency is a mark, and the low frequency is a space.

SITOR is used commonly on shortwave bands, where it is used to transmit maritime-related information such as weather forecasts and storm warnings.

SITOR-B is used for narrow-band direct printing (NBDP).^{[citation needed]}

NAVTEX marine weather and safety messages are broadcast using SITOR-B. The NAVTEX messages have a specific format that is interpreted by NAVTEX receivers. (NAVTEX is layered on top of SITOR-B just as HTTP is layered on top of TCP.)

Amateur radio uses SITOR but calls it AMTOR, AMateur Teleprinting Over Radio. AMTOR-A is SITOR-A. AMTOR-B (also called AMTOR-FEC) is SITOR-B. In 1991, an AMTOR extension was described that includes lower case and other printable ASCII characters.