Ciaran Barr, investment director at RPMI Railpen, will leave in the first half of this year, said a spokesman.
RPMI manages the assets of the £28 billion ($38 billion) Railways Pension Scheme, London.
Mr. Barr joined the firm in 2009 and held a number of positions, including head of strategy.
The spokesman said Mr. Barr has no firm plans at the current time. He added that RPMI is "well underway in the succession planning process and will make an announcement on (Mr. Barr's) successor in due course."