Harris, Swift best paid celeb pair
The Scottish DJ and Shake It Off singer are reported to be have earned 146 million dollars (£93 million) in the past year, thanks to successful music careers and lucrative advertising deals.
Harris, who has scored number one hits with Summer and Sweet Nothing, recently referred to Swift as "my girl" in a tweet congratulating her for changing "the entire music industry".
Apple changed its policy about payment to artists on their new streaming service after criticism from the 25-year-old songstress.
She has since agreed a deal to let Apple Music be the only service to stream her album 1989.
She told concertgoers at her recent gig in Glasgow: "I happen to love Scottish people, personally."
The pair were photographed embracing last month at the Billboard Music Awards in Las Vegas.
Beyonce and her husband Jay Z are said to have earned 110.5 million dollars (£70 million), and topped the list in 2013, falling to second place this year.
The full Forbes Celebrity 100 list, which will for the first time rank entertainers by their annual income only, is due to be released on Monday.
Swift and Harris are one of only three couples where both members have made the list.