Asda has been unveiled as the cornerstone outlet for the retail development behind Cardiff City FC's proposed 30,000-seat stadium.
The new stadium is planned for land near Ninian Park
The supermarket chain will move into a 92,000 sq ft site at the planned complex at Leckwith.
Developers called it a "watershed" for the £200m project.
Club owner Sam Hammam called it "fantastic news" after a "difficult and uncertain period" for fans, and it was hoped work could start later in 2006.
There has been uncertainty about the project, with Cardiff council giving the club three months to produce a business plan to prove its viability.
The US-owned cash-and-carry warehouse, Costco, has already taken the largest holding, of 135,000 sq ft, but Tuesday's announcement was seen as crucial to ensuring the planned retail development had enough backing to drive through the entire project.
Asda's share of the planned complex at Leckwith will comprise a 55,000 sq ft store with parking spaces, totalling 92,000 sq ft.
A spokesman for the firm said its planned store would be similar in size to the existing Ferry Road supermarket two miles away, and would be in addition to that and its two other stores in the city.
The stadium, which could rise to a 60,000 capacity, is planned for the other side of the road from its current Ninian Park ground, which will be redeveloped for housing by Redrow.
Tesco and Sainsbury
Securing retailers for 438,700 sq ft retail development on the 60-acre site was essential to fund the project.
Mr Hammam said on Tuesday: "This is fantastic news for the club and in particularly our many thousands of supporters. They have gone through a difficult and uncertain period and we have had their full support in driving forward our plan of the new stadium."
Paul Guy, of Cardiff-based developers PMG Estates, said the total stadium development was worth £200m but that the creation of the new road access - which will take 18 months - was now crucial to progress.
Tesco and Sainsbury had also bid for the site but the Asda proposal was seen as the "most reliable".
"I know there have been a lot of false dawns on this scheme but this is a watershed," said Mr Guy.
Rodney Berman, Cardiff Council leader said: "We now look forward to receiving the business plan from the club and working towards helping to deliver this ambitious, multi-use project in as short a time-scale as possible."
Mr Guy said he had seen the business plan and was confident that the club would convince the council of its viability.
It is hoped building will start by the end of the year.
The Premiership is 'on the horizon' for Bluebirds, says Peter Risdale
Leckwith Road will be widened to a dual carriageway over 18 months, with the scheme allowing for an extra access lane to become available on matchdays.
The council is currently insisting that the replacement for Leckwith athletics stadium is built before the new football ground is begun.
But, with the finance apparently in place, and the football club offering to pay for replacement transport for athletics clubs, the developers hope the council will agree to an early demolition of the Leckwith stand.
Negotiations between the football club and the council over the new stadium's business plan are to start next week with both sides hoping an agreement can be reached by March.
The club last season faced £30m debts and sold several players in an effort to reduce wages.
Club deputy chairman Peter Risdale said: "The Premiership is just over the horizon and we can hopefully see Premiership football coming to Cardiff in the not too distant future".
He said it would be possible that the Bluebirds would be playing in their new stadium in the 2008-2009 season, but would not be drawn on whether it would be for the start of that season.