The Symphony No. 9 (D major) by Gustav Mahler was written in 1909 and 1910. It was the last symphony that Mahler completed.

The piece is in four movements:

1. Andante comodo (D major)
2. Im Tempo eines gemächlichen Ländlers. Etwas täppisch und sehr derb (C major)
3. Rondo-Burleske: Allegro assai. Sehr trotzig (A minor)
4. Adagio. Sehr langsam und zurückhaltend (D flat major)

It is written for an orchestra made up of four flutes, piccolo, three oboes, cor anglais, an E flat clarinet, three B flat clarinets, a bass clarinet, four bassoons, a double bassoon, four French horns, three trumpets, three trombones, a tuba, timpani, glockenspiel, cymbals, bass drum, side drum, triangle, tambourine, three bells, two harps and strings (violins divided into two groups, violas, cellos and double basses).

Although the symphony has the traditional number of movements (four) it is unusual in that the first and last are slow rather than fast. As is often the case in Mahler, one of the middle movements is a ländler.

The first movement opens with a sighing motif, a reminder of the deep breaths of a man on his deathbed. Throughout the movement, the idea of breath remains with the listener.

The second movement is a dance, a ländler, but it has been distorted to the point that it no longer resembles a dance. It is reminiscent of the second movement of Mahler's Fourth Symphony in the distortion of a traditional dance into a dance of death. For example, Mahler alters traditional chord sequences into near-unrecognizable variations.

The third movement, in the form of a Rondo-Burleske, displays the final maturation of Mahler's contrapuntal skills. It opens with a dissonant theme in the trumpet which is treated in the form of a cyclical fugue . The movement is unique in the usage of dissonance treated with Baroque counterpoint.

The work ends quietly, fading away, and is often interpreted as being a self-conscious farewell to the world (Mahler died not long after its completion, and did not live to witness its premiere). However, as Mahler was working on his never-completed Symphony No. 10 at the same time as this one, this is perhaps unlikely. After all, Mahler scribbled a farewell to Music and Almschi in the manuscript of the Tenth.

The work was premiered on June 26, 1912 at the Vienna Festival by the Vienna Philharmonic Orchestra conducted by Bruno Walter. It was first published in the same year by Universal Edition.