Godswill Oyegwa Uyoyou is being sought by Nigeria's Economic and Financial Crimes Commission (EFCC).
A wanted notice claims he helped conspirators dressed as maintenance staff get into the bank so they could use computers to transfer cash.
Local reports suggest the theft was spotted when stolen cash was withdrawn.
Although no members of the gang have been caught, several are being "tracked", Wilson Uwujaren, a spokesman for the EFCC, told News Nigeria.
Details of the robbery are scant but it is thought that Mr Uyoyou and conspirators entered the bank on a Saturday when it was closed and no other staff were working.
The IT staffer was key to the robbery, said the EFCC, because of the access he enjoyed to the computer systems at the bank. This was used to siphon 6.28bn Nigerian Naira into accounts of the conspirators, said the EFCC. So far, the bank at the centre of the theft has not been named.
The EFCC has issued a warrant for Mr Uyoyou's arrest and he is being actively sought in Nigeria.
John Hawes, a computer security researcher at Sophos, said the amount of cash stolen was "unusually large" but the method the gang chose was "all too common".
"Insider risk is a major problem for banks," he wrote on the firm's security blog, "they still have to rely on trusted employees to behave themselves, resist temptation and keep their hands off the huge amounts of funds they may find themselves dealing with every day."