The retailer's Tu brand is the country's seventh largest clothing retailer with 7.5 million customers and sales of around £750 million last year.
It will begin its test of the online market with a pilot scheme selling products to an invited group of customers in the Midlands.
The market is already fiercely competitive, with store and online-based players including Next alongside dedicated internet operators such as Very.
However, Sainsbury's non-food trading director James Brown said the firm's fashion unit had doubled its design team, invested in the quality of its clothing and relaunched its Tu brand last year.
Mr Brown added: "Now's the right time to explore the online channel as a complement to our store business."
Sainsbury's online director Robbie Feather added that the pilot would allow the retailer "to work with a group of customers to build the right customer experience."
Sainsbury's said it will offer customers involved in the test a bespoke website and mixture of click and collect and home delivery services.
The retailer, which first began selling clothes in 1994, said orders would be handled from its clothing depot in Bedford, which also serves 400 stores.