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SOS is the international Morse code distress signal. It consists of three dots, three dashes and three dots (···---···). This is the same as for the letters S, O and S, except that it is sent as one single letter - that is, with the inter-letter spaces converted to inter-element spaces.

Although thought by some to mean Save Our Souls or Save Our Ship, in actuality the signal was agreed upon because it was easy enough for even an amateur to use or recognize, even with interference.

The original radio distress signal was CQD. Proposed by Marconi and adopted in 1904, the CQ was a general attention notice with the D for distress. It was popularly rendered as "come quick, distress". The CQD signal did not last long; at an international conference in Berlin in 1906 the German standard SOS was adopted and officially ratified in 1908. The CQD signal remained in use for a few more years, especially with British operators who had first proposed it.

The first significant rescue following a radio distress signal was in January 1909 when 1500 people were recovered from the collision of the Republic and the Florida by the Baltic after it had heard the CQD message.

The most famous first major use of SOS was with the sinking of The Titanic on April 15, 1912, when the radio operators used it in conjunction with the older distress signal.

Urgent but not emergency calls used the prefix XXX.

See also distress signal and call for help

Shit on a shingle (S.O.S.) is also an abbreviated military slang term for chipped beef on toast, an infamous dish as served to the U.S. Army.

There is a brand of detergent by the name of SOS, supposed to stand for "Save Our Saucepans."

Son of Sevenless (SOS) is an exchange factor that stimulates GDP release from Ras to allow GTP binding and Ras to be activated. Proline-rich regions of SOS bind to the Src-homology 3 (SH3) domains of Grb2. Phosphorylation of receptor or adaptor proteins on certain Tyr residues results in binding to the SH2 domain of Grb2, thereby recruiting the Grb2-SOS complex to the membrane where Ras resides. SOS also has a pleckstrin homology (PH) domain and dbl homology domain. The dbl homology domain can stimulate GDP/GTP exchange on Rac, and the PH domain regulates the Rac exchange activity and mediates binding to the membrane.