The creative network was appointed to work across 41 markets after a process run through Creativebrief.
The appointment does not affect the brand's global ad agency, 72andSunny.
New work is expected to launch in 2018.
Starbucks called a review during the summer for its first specialist ad agency to work across the EMEA region.
At the time, the brand said it wanted an agency to help it "celebrate how Starbucks' coffee leadership comes to life across a diverse region".
Maria Sebastian, senior vice president, brand for Starbucks EMEA, described Iris as: "a best-in-class creative agency who demonstrated during this process the opportunity we have to celebrate our coffee leadership, community values, and tell our brand story, across such a fantastically diverse region."
The coffee chain also works with Oliver, which provides an in-house unit that manages in-store and point-of-sale activity.