A filing with the US Securities and Exchange Commission issued on Tuesday showed ESL Investments now holds 24.8 million Citigroup shares, valued at $1.27 billion.
At the end of the first quarter the Connecticut-based firm had held 15.2 million shares worth $783 million.
Mr Lampert's $15 billion fund also holds stakes in Sears Holding, Motorola, AutoNation and AutoZone.
News of the purchase of Citigroup shares comes as speculation has been growing that Mr Lampert's funds may be suffering rare losses this year.
Since establishing the fund in 1988, Mr Lampert, who is one of the world's top-ranked investors, has only lost money on two occasions - in 1990 and 2002.
Mr Lampert's strategy has been marked by investing in only a small number of companies at a time.