"M" is a song written by Japanese singer Ayumi Hamasaki from her album I Am... (2002). The lead single from the album and Hamasaki's nineteenth overall, "M" marked Hamasaki's increased creative control over her music, as it was the first song she composed, under the pseudonym "CREA".

The single is to date one of Hamasaki's most commercially successful; it peaked at the top spot on the Oricon weekly charts, selling over 500,000 units on its first week. Moreover, the single eventually sold over 1,000,000 copies over its seventeen-week run and became an RIAJ-certified million-seller; "M" also won the Japan Gold Disc Award for "Song of the Year".

Shortly after the release of her studio album Duty, Hamasaki began writing "M". Before then, Hamasaki's staff had composed the melodies; Hamasaki only wrote the lyrics. However, with "M", Hamasaki felt that none of the melodies composed by her staff fit her vision of the song. Consequently, she decided to compose the melody herself. She began work on an electronic keyboard; however, as she had little proficiency in the instrument, Hamasaki eventually resorted to singing the melody into a voice recorder.

Throughout the song, Hamasaki addresses "Maria". Hamasaki is ambiguous as to the identity of "Maria"; however, she has stated that the song was inspired by a story told to her by a friend about a saint named Mary. The story was pivotal in the shaping of the theme of the song, which, according to Hamasaki, is about a "woman who won't change with time". Finally, like other songs from I am..., Hamasaki explores the topics of couples and love.

According to Hamasaki, the melodies composed by her staff for "M" did not fit her image for the song because they were too "warm"—she had envisioned a melody with a "cold" feeling. That, according to Hamasaki, meant a melody that was "difficult to grasp" and started in a lower key before progressing to a higher key. The song is written in common time and begins in the key of C major before progressing to the key of C-sharp major. The song uses piano, electric guitar, triangle, and various stringed instruments.

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