The singer - who is dating Fall Out Boy rocker Pete Wentz - has teamed up with US store Wet Seal, which will sell the tops in conjunction with the release of her third album 'Bittersweet World'.
She said: "I was inspired by so many fun, wonderful things as I was recording 'Bittersweet World' and it's been great to carry through those inspirations into these new shirt designs. I hope my fans love this album and rock out in these shirts!"
Wet Seal is a young women's clothing retailer which stocks moderately priced brand name and company-designed apparel and accessories.
Ashlee is following in the footsteps of her older sister Jessica, who released her own fashion range in 2005.
Other stars with their own clothing lines include Victoria Beckham, Kate Moss, Jennifer Lopez and Mary-Kate and Ashley Olsen.
Earlier this week, it was revealed ageing rocker Rod Stewart had also launched his own fashion range.
The 'Maggie May' singer designed a range of items including T-shirts, tracksuit tops, baby bibs and official 'Rod Stewart Panties', which have love hearts and 'Isn't it romantic?' emblazoned across the front.
Ashlee Simpson is set to design her own range of T-shirts for teenage girls.
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Jessica Simpson and boyfriend Tony Romo have proved they are still an item--by putting on a display of affection at her sister's weekend wedding. 
A source close to the couple told America's OK! magazine: "Ashlee and Pete are as perfect for each other as any two people could be.The 'Autobiography' singer - who recently announced her engagement to Fall Out Boy rocker Pete Wentz - is said to be expecting her first child in October.
Hotel heiress, master self-promoter and aspiring pop-music star Paris Hilton (its first hit is "Stars are Blind") will issue her first album on Aug. 22, two weeks later than previously announced.The self-titled set was made with the help of some of the hottest producers money can buy, including Scott Storch (50 Cent, Beyonce), Jonathan "J.R." Rotem (Rihanna) and Dr. Luke (Pink, Kelly Clarkson).