Asos said the service will be powered by its Barnsley warehouse colleagues.
Asos Instant will be initially be available to customers in any of London’s 122 postcodes, on orders placed before 10am from Sunday to Friday.
The service will cost £12.95 per order, which Asos said is in line with other same day delivery prices. Parcels will be delivered between 6pm and 10pm on the same day. Availability in other major cities around the UK will follow in the coming months.
Asos, whose high-profile fans include singer Rita Ora and former US First Lady Michelle Obama, has its main warehouse in Barnsley and is the town's biggest employer.
It is investing £23m in the warehouse this year.
The investment includes relaxation spaces, as well as the creation of a well-being centre for staff and new office space.
The warehouse, which employs around 4,000 people, was recently awarded ISO 9001/14001/18001 accreditation for exceptional management, environmental management and health & safety.
It was also awarded an ROSPA Gold Award for workplace health and safety.
Asos describes itself as a global fashion destination for 20-somethings, selling cutting-edge fashion.
It sells over 85,000 branded and own-label products to almost every country in the world.