The International Radiotelephony Spelling Alphabet or simply the Radiotelephony Spelling Alphabet, commonly known as the NATO phonetic alphabet, is the most widely used set of clear-code words for communicating the letters of the Roman alphabet. Technically a radiotelephonic spelling alphabet, it goes by various names, including NATO spelling alphabet, ICAO phonetic alphabet, and ICAO spelling alphabet. The ITU phonetic alphabet and figure code is a rarely used variant that differs in the code words for digits.

Although spelling alphabets are commonly called "phonetic alphabets", they are not phonetic in the sense of phonetic transcription systems such as the International Phonetic Alphabet.

To create the code, a series of international agencies assigned 26 clear-code words (also known as "phonetic words") acrophonically to the letters of the Roman alphabet, with the goal that the letters and numbers would be easily distinguishable from one another over radio and telephone. The words were chosen to be accessible to speakers of English, French and Spanish. Some of the code words were changed over time, as they were found to be ineffective in real-life conditions. In 1956, NATO modified the then-current set used by the International Civil Aviation Organization (ICAO); this modification then became the international standard when it was accepted by ICAO that year and by the International Telecommunication Union (ITU) a few years later.

The 26 code words are as follows (ICAO spellings): Alfa, Bravo, Charlie, Delta, Echo, Foxtrot, Golf, Hotel, India, Juliett, Kilo, Lima, Mike, November, Oscar, Papa, Quebec, Romeo, Sierra, Tango, Uniform, Victor, Whiskey, X-ray, Yankee, and Zulu. ⟨Alfa⟩ and ⟨Juliett⟩ are spelled that way to avoid mispronunciation by people unfamiliar with English orthography; NATO changed ⟨X-ray⟩ to ⟨Xray⟩ for the same reason. The code words for digits are their English names, though with their pronunciations modified in the cases of three, four, five, nine and thousand.

The code words have been stable since 1956. A 1955 NATO memo stated that:

It is known that [the spelling alphabet] has been prepared only after the most exhaustive tests on a scientific basis by several nations. One of the firmest conclusions reached was that it was not practical to make an isolated change to clear confusion between one pair of letters. To change one word involves reconsideration of the whole alphabet to ensure that the change proposed to clear one confusion does not itself introduce others.