SOS is a Morse code distress signal ( ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄ ), used internationally, originally established for maritime use. In formal notation SOS is written with an overscore line (SOS), to indicate that the Morse code equivalents for the individual letters of "SOS" are transmitted as an unbroken sequence of three dots / three dashes / three dots, with no spaces between the letters. In International Morse Code three dots form the letter "S" and three dashes make the letter "O", so "S O S" became a common way to remember the order of the dots and dashes. IWB, VZE, 3B, and V7 form equivalent sequences, but traditionally SOS is the easiest to remember.

SOS, when it was first agreed upon by the International Radio Telegraphic Convention in 1906, was merely a distinctive Morse code sequence and was initially not an abbreviation. Later a backronym was created for it in popular usage, and SOS became associated with mnemonic phrases such as "save our souls" and "save our ship". Moreover, due to its high-profile use in emergencies, the phrase "SOS" has entered general usage to informally indicate a crisis or the need for action.

SOS originated in German government maritime radio regulations adopted effective 1 April 1905. It became a worldwide standard when it was included in the service regulations of the first International Radiotelegraph Convention signed on 3 November 1906, which became effective on 1 July 1908. In modern terminology, SOS is a Morse "procedural signal" or "prosign", used as a start-of-message mark for transmissions requesting assistance when loss of life or catastrophic loss of property is imminent. Other prefixes are used for mechanical breakdowns, requests for medical assistance, and a relayed distress signal originally sent by another station. SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System.

SOS is still recognized as a standard distress signal that may be used with any signaling method. It has been used as a visual distress signal, consisting of three short/three long/three short flashes of light, such as from a survival mirror. In some cases the individual letters "S O S" have been spelled out, for example, stamped in a snowbank or formed out of logs on a beach. "S O S" being readable upside down as well as right side up (as an ambigram) is an advantage for visual recognition.

Radio (initially known as "wireless telegraphy") was developed in the late 1890s, and was quickly recognized as an important aid to maritime communication. Previously, seagoing vessels had adopted a variety of standardized visual and audio distress signals, using such things as semaphore flags, signal flares, bells, and foghorns. However, cooperation in standardizing radio distress signals was initially limited by national differences and rivalries between competing radio companies.

In 1903, an Italian representative at the Berlin Preliminary Conference on Wireless Telegraphy, Captain Quintino Bonomo, discussed the need for common operating procedures, including the suggestion that "ships in distress ... should send the signal SSS DDD at intervals of a few minutes". However, procedural questions were beyond the scope of this conference, so no standard signal was adopted at the time, although Article IV of the conference's Final Protocol stated that "Wireless telegraph stations should, unless practically impossible, give priority to calls for help received from ships at sea".

Without international regulations, individual organizations were left to develop their practices. On 7 January 1904 the Marconi International Marine Communication Company issued "Circular 57", which specified that, for the company's worldwide installations, beginning 1 February 1904 "the call to be given by ships in distress or in any way requiring assistance shall be C.Q.D." An alternative proposal, put forward in 1906 by the U.S. Navy, suggested that the International Code of Signals flag signals should be adopted for radio use, including NC, which stood for "In distress; want immediate assistance".

Germany was the first country to adopt the SOS distress signal, which it called the Notzeichen signal, as one of three Morse code sequences included in national radio regulations which became effective on 1 April 1905. In 1906, the first International Radiotelegraph Convention met in Berlin, which produced an agreement signed on 3 November 1906 that become effective on 1 July 1908. The convention adopted an extensive collection of Service Regulations, including Article XVI, which read: "Ships in distress shall use the following signal:  ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄  repeated at brief intervals".