The songstress, 31, officially revealed the big news during a music festival in Mexico on Sunday.
She struck a pose in the reflection of a mirror while joined by friend and model, Georgia May Jagger.
The actress donned a glittering, purple minidress and flashed an excited smile with her stomach on display.
She layered the dazzling look with a fuzzy, pink jacket and also opted for sparkling, gold bottoms for a finishing touch.
Jagger stood close behind Waterhouse as the fashionable pair paused for the quick photo inside what appeared to be a dressing room.