Barr, who will join the John Lewis board, is currently acting marketing director, reporting to commercial director Phil Hullah.
Hullah will retain the title of commercial director with responsibility for John Lewis Direct, and will take on his new role from June 18.
Barr joined John Lewis in January 2007. She was formerly chief executive of Deliverance Limited and business development director at Woolworth, having previously held positions with Kingfisher, KPMG and Freemans.
She will join the John Lewis management board from June 4.
Andy Street, managing director of John Lewis, said: "Over the last few years, John Lewis has brought the customer perspective to the heart of its planning and activity.
"The development of our marketing capability has been one of the essential factors in driving this move and cementing our position as Britain's favourite retailer. The new board role of marketing director will be key in supporting our plans for substantial growth over the next decade."
Barr said: "We will continue to communicate with customers and develop the John Lewis brand. I look forward to evolving our plans and building long-term relationships with all our agencies."
Lowe London holds the creative account for the retailer, while media planning and buying is handled by Omnicom shop Manning Gottlieb OMD, which also handles the media account for sister retailer Waitrose.
Direct marketing is handled by independent agency Kitcatt Nohr.