The investor made the revelation during a speech at the Ira Sohn investment research conference.
Mr Ackerman, a manager at the Pershing Square Capital Management hedge fund, made the comments as an aside during a talk which, according to Market Watch, focused on the future of credit rating agencies.
He was quoted by Reuters as saying: âAnd by the way, we bought about 150 million shares of Citigroup, but I don't have time to talk about it."
The fund manager did not provide any further comments to support his claim.
Meanwhile, the US Treasury recently announced it made $6.2 billion from selling 1.5 billion of the 7.7 billion shares it owns in Citigroup.
The American government is selling the stock as part of a wider plan to recoup some of the costs it incurred through bailing out the bank during the global credit crisis.
Citigroup borrowed $45 billion through the governmentâs Troubled Asset Relief Program.
Sales agent Morgan Stanley has been requested to organise the sale of a further 1.5 billion shares in the bank.
By Jim Ottewill