The pair, who brought 50 per cent of the Upton Park club from Icelandic group CB Holdings in January, have now increased their shares from 25 per cent each to 30 per cent.
Sullivan and Gold say half the money will go to CB Holdings and the other half to ease the Hammers’ financial plight.
Chief executive Karren Brady says the deal will give the club “the much-needed stability that it needs”.
The two men also now have a three-year option to buy the remaining 40 per cent of the club from CB Holdings, in which stricken Icelandic bank Straumur has the majority interest.
“Myself and David Gold are, like you, West Ham United fans and our aim is to ensure our club enjoys a stable and successful future,” Sullivan told the club website.
“We have a long-term intention of increasing our shareholding, while we have also invited professional investors to share in our vision for this special club. Also, in the near future we hope to extend to supporters the opportunity to invest in the club.”
Brady, meanwhile, insists captain Scott Parker is not for sale despite admitting the club still has “an enormous amount of debt”.
Parker is being linked with a £10m bid from Aston Villa but Brady said: “David Sullivan has given his word that Scott Parker is not going to be sold. He is Player of the Year and is very important to us.”