The launch is timed to coincide with what is expected to be the busiest online shopping day of the year, dubbed Mega Monday.
Amazon, which reportedly considered a bid for Asos, is trying to find ways to replicate its success in books and technology with fashion.
The new fashion store is understood to have signed up brands including 7 For All Mankind, Pringle, and Black Orchid.
Amazon sold more than three million items and took 35 orders per second on Mega Monday last year.
According to estimates by Visa Europe, £320 million will be spent online in total today, which it predicts will be the busiest online shopping day ever in the UK.
Marks & Spencer has recruited an additional 800 employees at its UK distribution centres and doubled the number of staff in its e-commerce call centre.
Asos, which targets twenty-somethings with clothes based on outfits first worn by celebrities, reported a 42% leap in underlying profits to £13.2 million for the five months to August 31, having recently changed its year end from March 31.
Asos, which stands for As Seen On Screen, recently celebrated reaching the five million customer mark and confirmed it had more international customers than those in the UK.