The US singer was pictured strolling through West Hollywood with the up and coming actor, who is the son of superstar Diana Ross.
Ashlee, 29, kept it casual but stylish by teaming a Bob Marley and The Wailers T-shirt with a pair of black skinny jeans, a burgundy and cream boho cardigan and a pair of nude ballet pumps.
She kept her long blonde hair loose and topped off her ensemble with a funky beige trilby hat.
Evan, meanwhile, who will star in forthcoming movie The Hunger Games: Mockingjay - Part 1, also looked low-key in a tee, black jeans, chunky boots and a pair of aviators.
Ashlee, who is the sister of Jessica Simpson and mum to son Bronx, is thought to have got together with 25-year-old Evan in June.
In July, Ashlee and Evan made their relationship public by attending one of Jessica's fashion launches together in New York.
Since then, the couple haven't shied away and wowed on the red carpet for the LA premiere of Hunger Games: Catching Fire in November.
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