Global has worked with Rocket since June, when Giles Pearman was appointed marketing director. The radio group had ended its relationship with Publicis Groupe's MediaVest in April.
When Pearman was appointed, Global Radio said he would handle its media agency review, but Rocket has been appointed permanently without a pitch.
Global Radio spent £6.4m on media in the year to 30 September, of which £2.17m was on TV and £2.33m on radio, according to The Nielsen Company.
Capital FM, one of the Global stations, is embarking on a multimedia campaign to promote its Jingle Bell Ball with Microsoft Windows 7.
TV ads will air around shows such as The X Factor, while press ads will run in papers including London Lite and Metro. Online, the campaign will include activity on Facebook and Google, and outdoor work will run in Tube stations.
The TV ad, voiced by breakfast presenters Johnny Vaughan and Lisa Snowdon, will run for five weeks in the build-up to the sold-out event on 5 and 6 December at London's O2 Arena.