The ground, which has hosted Rugby World Cup matches, was sold on a lease-back deal for £1.25million in 1997.
The buy-back will boost a proposed £20million development near the Trostre tinplate works, and will be a huge boost to the Scarlets regional team.
The new stadium development is a joint venture with Carmarthenshire County Council and will signal the end of nearly 125 years of rugby at Stradey Park.
Chief executive Stuart Gallacher said: "The buy-back of Stradey was critical to the whole scheme and now it has been achieved we are another step closer to our new home."
