West Ham's owners David Gold and David Sullivan loaned the club £35million to help ensure their immediate return to the Barclays Premier League.
The significant figure was revealed in the club's newly-released accounts, which covered the 12 months they were in the npower Championship.
"Both myself and my partner David Gold decided to invest our money into achieving promotion and during the season we invested a further £32.2million of our cash in order to allow the manager to go into the transfer market and buy the players we needed to secure promotion," Sullivan wrote in his chairman's statement..
"During that period we brought or loaned 18 players, most notably Kevin Nolan, Matthew Taylor and Ricardo Vaz Te, investing a total of £17.1m."
The club revealed losses of £25.5m while they were in the second tier.
Turnover during that period, which followed relegation, fell by £34.4m to £46.25m although Sullivan said that was "inevitable" following the loss of TV and sponsorship income totalling £26.5m.
Since earning promotion West Ham have spent a further £13m on players, a figure that could rise to £20m.
Gold also re-confirmed talks with manager Sam Allardyce over a new deal would only take place at the end of the season.
Despite that he expects Allardyce, whose contract expires in the summer, will remain for next season.
"I'm very optimistic that Sam will be our manager next season. But we have to sit down," he said. "The agreement with the board was that we would sit down at the end of the season not when we're safe - we are not mathematically safe as we speak."