Walker will join the board from 1 September as a non-executive director, with Agius stepping down from his dual role as director and chairman from 31 October.
Agius stayed on following the resignation of former chief executive Bob Diamond in the aftermath of the Libor scandal that swamped Barclays in early July, in order to help bridge the gap left at the head of the institution until replacements could be found.
In addition to his current role as senior adviser to Morgan Stanley International, Walker has held a number of high profile positions over the past 30 years.
These have included: assistant secretary at the Treasury, executive director at the Bank of England, chairman and chief executive of the Securities and Investment Board; deputy chairman at Lloyds Bank and vice chairman of Legal & General.
Since 2007 Walker has completed two independent reports and made recommendations regarding the private equity industry and corporate governance at financial institutions.
He also co-led the independent review of the report the FSA produced into the failure of Royal Bank of Scotland.
Agius said of the appointment: “Barclays is fortunate to have attracted Sir David Walker to succeed me as Chairman. He will be taking over at a time when Barclays universal banking model is delivering a strong performance in difficult markets. I wish him every success as he leads Barclays at this important time.”