Jamie George has been in charge since the 2023 World Cup but, after a disappointing year in which England won only five of their 12 Tests, he has been replaced by his Saracens team-mate.
It is the first time Itoje has been give the role, although he took over from George when the hooker was substituted in games, and the 30-year-old continues his rise as a leader having been installed as Saracens skipper at the start of the season.
"Maro's a world-class player who has the respect of everybody so I feel really privileged to have the opportunity to appoint Maro as captain," Borthwick said.
Borthwick's predecessor as England boss, Australia's Eddie Jones, doubted Itoje would ever be captain, describing him as "very inward-looking", but the two-time British and Irish Lions tourist has proved him wrong.
"With the role comes a great deal of responsibility, but I'm at a point in my career where I feel ready to give my all to serve the team and the fans with the captaincy, and also produce my best on the field," Itoje said.
The news came as a blow to George, 34, a veteran of 97 caps.
"Jamie's disappointed, as anybody would be," Borthwick said. "One of Jamie's great strengths as a leader, person and player is that he's always put the team first. Having spoken to him, that's what his intent is."
Borthwick has picked a 36-man squad for the Six Nations that includes rampaging Saracens number eight Tom Willis, but is missing veteran prop Dan Cole and scrum-half Ben Spencer.
ENGLAND SIX NATIONS SQUAD:
Forwards: F Baxter, O Chessum, A Coles, L Cowan-Dickie, C Cunningham-South, B Curry, T Curry, T Dan, A Dombrandt, B Earl, E Genge, J George, J Heyes, T Hill, M Itoje (capt), G Martin, A Opoku-Fordjour, B Rodd, W Stuart, T Willis.
Backs: O Beard, E Daly, F Dingwall, G Ford, T Freeman, O Lawrence, A Mitchell, C Murley, H Randall, T Roebuck, H Slade, O Sleightholme, F Smith, M Smith, F Steward, J van Poortvliet.
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