Internet radio pioneer
Westergren will replace Brian McAndrews as chief executive of the Oakland-based company. McAndrews was named CEO in 2013.
Pandora's shares fell $1.03, or 9%, to $9.90 in midday trading on Wall Street after Pandora announced the change in its leadership.
Westergren first served as CEO from 2002-04, and has remained involved with the company since. Pandora is in the midst of a turnaround plan, having made recent acquisitions including the assets of failed streaming rival Rdio and online ticket seller Ticketfly.
Follow Ryan Faughnder on Twitter for more entertainment business coverage: @rfaughnder