Mark Sharman, head of marketing and comms at the information regulator - the combined body for ruling on data protection and freedom of information issues - said the appointment was made following a three-way pitch. He declined to reveal which other agencies had taken part.
The account will be reviewed after six months with a view to establishing a three-year rolling contract.
The post of Information Commissioner was created after the Freedom of Information Act was passed in March 2000.
One priority for Citigate will be to raise the organisation's profile, particularly as Richard Thomas will be taking over from Elizabeth France as Information Commissioner in December.
The agency appointment comes as the Information Commissioner gears up for a series of comms challenges - including preparing for the Freedom of Information Act which comes into force in 2005.
Under the Act, local authorities and other public bodies must be able to supply a range of information to the public on demand.
The organisation also has responsibility for enforcing data protection codes and educating the public about their rights under those codes.
At the same time the organisation is running a publicity campaign to counter a spate of frauds which have been aimed at data controllers. Criminals have been fraudulently telling controllers they can meet new legal obligations by paying a fee of around £100.
The four-strong Citigate account team will be headed by director Nick Stace, who said work on the account had begun.