The alto saxophone is a variety of the saxophone, a woodwind instrument. The alto is the fourth in size of the sax family which consists from smallest to largest of sopranissimo (or soprillo), sopranino, soprano, alto, tenor, baritone, bass, contrabass, and subcontrabass. Of these, only the soprano through baritone are commonly used. The less tubing an instrument has, the higher the sounds it makes.

The range of most alto saxophones is from low concert D flat to high concert A flat, with higher-end Altos having an extra key to go to high A.

The alto saxophone is a transposing instrument and reads the treble clef in the key of E flat (that is to say, a written C will sound like an E flat).

Great players of the alto saxophone include Charlie Parker, Cannonball Adderley, and Sonny Stitt. The alto saxophone is included in concert music more often than the tenor, and many great concert solos for alto exist. It has great versatility and is used commonly in concert, jazz, funk, blues, pop, and rock music. Most people who learn to play saxophone begin on alto. The alto and tenor saxophones are the most commonly played. Altos range in price from approximately $900-$3000 USD.