The couple were rumoured to have been an item for a number of weeks now, but appeared to officially confirm that they are dating on Tuesday (30 July) by making their first public appearance together at Ashlee's older sister Jessica's fashion line launch in New York.
28-year-old Ashley hosted the launch on behalf of her sister - who recently gave birth to her second child, Ace Knute - and actor Evan was her dashing date.
And now it seems as though the new couple can't stay away from one another, as they walked through LAX airport hand in hand yesterday (31 July).
Both stars looked cool in their similar outfits and hid behind circular framed sunglasses.
While singer Ashlee wore baggy grey tracksuit bottoms with heels and a cute leather jacket, 24-year-old Evan kept things ultra simple in a black top, faded black jeans and black biker boots.
Although the two lovebirds have remained tight lipped on their romance, Evan - who is the son of legendary singer Diana Ross - appears to be head over heels for Ashlee judging by his Twitter profile.
The young actor has posted a few of pictures of his new girlfriend on the social network and seems to be ever the supportive partner by championing her current role as Roxie in the stage production of Chicago.
One tweet read: "2nd night watching #chicago at the Hollywoodbowl. @ashleesimpson was incredible again and the rest of the cast as well was very #inspiring"
He also posted a photo that shows him leaning in towards Ashlee as they both sit outside on a step. He simply captioned it: "@ashleesimpson #cabohippylife"
Ashlee was previously married to Fall Out Boy's Pete Wentz and the couple share a child together, four-year-old Bronx Mowgli Wentz.
What do you think of Ashlee and Evan as a couple? Let us know below, as the best comment each week wins £50!
Babies, divorces and yoga: yesterday's top showbiz stories in pictures