Cook has played 36 Tests and scored 2,694 runs at 42.09
England opener Alastair Cook will be Andrew Strauss' vice-captain in the four-Test series against West Indies, which starts on 4 February.
Strauss was appointed captain earlier this month following the resignation of Kevin Pietersen, and Cook has long been viewed as a future England skipper.
"It's a very unofficial position but is a chance to gain some valuable leadership experience," said Cook, 24.
"Straussy will need support not just from me but from other senior players."
Andrew Flintoff or Paul Collingwood might have been expected to fill the role of Strauss' right-hand man, given their previous captaincy experience.
All-rounder Flintoff has led England in 11 Tests, most notably on the disastrous tour of Australia in 2006-2007.
And Collingwood captained England in 24 one-day internationals before giving up the job last year.
However BBC cricket correspondent Jonathan Agnew said the unofficial appointment showed England have "one eye on the future".
Indeed, Essex batsman Cook is hoping his relative youth will bring a fresh perspective to the role.
"We have a lot of leadership experience in the dressing room with three other guys who have captained England," said Cook.
"They will be there to offer help and ideas and I can give them the viewpoint of someone that is slightly younger and might come at things from a different angle."