Hewat, who has been an instrumental commercial driver at TMG since June 2009, is believed to be starting his new role at GNM’s Kings Place in the summer of 2013, although this has yet to be confirmed.
As commercial director, Hewat will report to David Pemsel, who became chief commercial officer at GNM last October.
Hewat will have overall responsibility for GNM's commercial sales operations, managing key relationships with clients and agencies, and translating the Guardian's market-leading reach, cross-platform advertising solutions and long-term brand partnerships into sustainable revenue.
He said he was "looking forward" to his new role, and called The Guardian "one of the world's strongest editorial brands", which has a "compelling story to tell advertisers and agencies".
At TMG, Hewat has overseen a sales team of 40, responsible for TMG's newspapers and magazines, as well as its Manchester and international sales operations.
Prior to his time at the Telegraph, Hewat was the commercial director of Virgin Radio (now Absolute Radio).
Pemsel said: "It is a testament to the strength and scale of our brand that we are able to welcome someone of Nick's calibre to the Guardian.
"The wealth and breadth of experience that Nick has to offer will help bring new focus to the Guardian's revenue-generating efforts, as well as driving home the commercial value we, as the UK's leading quality news brand*, can provide our clients."
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