Standard & Poor's (S&P) president Deven Sharma has stepped down just weeks after the agency downgraded the US credit rating.
He will be replaced by Douglas Peterson, chief operating officer of Citibank with effect from 12 September, the agency said.
Mr Sharma will take on "a special assignment" within the company before leaving at the end of the year.
S&P has been criticised by US authorities for the credit rating cut.
Timothy Geithner, the US Treasury Secretary, in an interview with NBC said the agency had shown terrible judgement and handled themselves poorly.
However, the company did not mention whether the decision was related to the credit rating downgrade and its criticism thereafter.
"S&P will continue to produce ratings that are comparable, forward looking and transparent." the company said in a statement.