Apple CEO Tim Cook tweeted, "Thanks @Pharrell for an unforgettable Earth Day celebration!", alongside a photo of him on stage.
Pharrell was first spotted with an Apple Watch weeks ago when he teased the device on The Voice and Instagram.
Apple kicked off this week with its environmentally responsibility report as well as a video highlighting its sustainability efforts. Today, in celebration of Earth Day, Apple added a green leaf to their logo at retail stores.
While Apple is celebrating the environment this week, it is also gearing up for the Apple Watch launch on April 24.