The review was launched in March as the high-street fashion chain announced that it planned to further expand its operations globally.
Universal McCann won the business after a three-stage pitch process among several leading unnamed London agencies.
H&M, which has 945 stores worldwide, plans to open 140 new stores globally, including five in Canada and its first store in Slovenia. It currently operates in 18 countries, mainly in Europe.
H&M spent £25m globally on media last year and £2.5m in the UK. It is understood that the review was being conducted market by market.
Andy Jones, joint managing director of Universal McCann, said: "H&M is a fantastic brand and we are delighted to be reappointed after a vigorous review process. It's further proof of Universal's outstanding retail and brand credentials and H&M knows we can continue to deliver for such a prestigious client."
H&M's biggest markets are the UK, Germany, Sweden and Norway. Its advertising has so far concentrated on press and poster activity and featured the likes of Jade Jagger, Tim Roth and Kate Moss.
An outdoor campaign featuring supermodel Heidi Klum for H&M was so popular this year that fans were ripping posters off walls and damaging hoardings to get at the pictures underneath.
Some of the ads were being put up for sale on auction site eBay and at one point were changing hands for £20.
The chain's current positioning, marketing "fast fashion" at low prices, competes with the likes of New Look, Zara and Top Shop in the UK.
McCanns first won the business in 2000 from CIA Conzept. The appointment bolsters the agency's new-business record this year, which also includes RHM Foods and the Telegraph.
The pitch was led by Jones and fellow joint managing director Damian Blackden.
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