The ‘Asda price guarantee' aims to specifically up the ante against rivals Tesco, Sainsbury's and Morrisons, and forms part of the retailer's plans to retain customers.
The new initiative is being driven by a print campaign in national newspapers and a TV ad, created by Saatchi & Saatchi, which airs today.
The UK's second biggest supermarket will use the web for the first time to guarantee that its prices cannot be beaten.
A new site enables shoppers to type in their receipt and check to see if their shop of eight or more products would have been cheaper at Tesco, Morrisons or Sainsbury's.
If it is found to be cheaper elsewhere, Asda will refund the difference in the form of a voucher which can be redeemed in-store, plus one pence back for ‘good measure'.
However, Asda has conceded around 30% of its products will not be included in the price checker, as they are either unique to the store or are subject to strict trading standards regulations.
Customers will also not be able to claim a refund for more than £25 in any month or make more than 10 claims in any month per household.
Asda's outgoing chief executive Andy Bond claimed the guarantee put an "end to the phoney price wars that most shoppers are sick of. Plain and simple, our Price Guarantee is our cast-iron promise that your basket of shopping will always be cheapest at Asda".
The scheme comes as the supermarket is being forced to respond after recording the slowest sales growth of the big four supermarkets for the fourth consecutive month.
Data from Kantar Worldpanel, the consumer research group, found that Asda had the lowest sales growth of only 2.5% in the 12 weeks to 18 April. Consequently, its market share slipped from 17% to 16.9%.