An arranger is a person who creates a musical arrangement or adaptation of a work. Arrangement refers to the adaptation of an existing piece of music to a different instrumentation or scoring than the original version. The arranger thus plays an important role as a co-creator of a composition.

An arranger takes the existing musical structure of a piece and adapts it to provide a different sonic presentation or interpretation. This may mean arranging the work for different instruments or ensembles to suit the sound and mood of the piece. For example, an arranger might rewrite an orchestral composition for a jazz band or arrange a pop song for a string quartet.
The arranger works closely with the composer or creator of the work to ensure that the original intent of the piece is preserved while adapting it to the specific requirements of the new instrumentation or scoring. Various musical elements such as melody, harmony, rhythm and structure may be reinterpreted and transposed.