This paper describes AutoMashUpper, an interactive system for creating music mashups by automatically selecting and mixing multiple songs together. Given a userspecified input song, the system first identifies the phraselevel structure and then estimates the “mashability” between each phrase section of the input and songs in the user’s music collection. Mashability is calculated based on the harmonic similarity between beat synchronous chromagrams over a user-definable range of allowable key shifts and tempi. Once a match in the collection for a given section of the input song has been found, a pitch-shifting and time-stretching algorithm is used to harmonically and temporally align the sections, after which the loudness of the transformed section is modified to ensure a balanced mix. AutoMashUpper has a user interface to allow visualisation and manipulation of mashups. When creating a mashup, users can specify a list of songs to choose from, modify the mashability parameters and change the granularity of the phrase segmentation. Once created, users can also switch, add, or remove sections from the mashup to suit their taste. In this way, AutoMashUpper can assist users to actively create new music content by enabling and encouraging them to explore the mashup space.