The deal is a key win for RealNetworks over its archrival Microsoft, as both companies try to expand from the desktop into the wireless services market.
"If Microsoft had won this contract, the battle for the mobile media player market would have been close to being over," said Ovum analyst Roger Entner, who noted that Microsoft is ahead of RealNetworks in the desktop media player market.
"This is version 2.0 of the media player battle. It shows that somebody is fighting back," Entner said.
Cingular's biggest rival, Verizon Wireless,that delivers news and entertainment video clips to customers using its high-speed wireless data network.
Like other operators around the world, Cingular and Verizon are developing advanced services such as mobile video and music in a bid to boost revenue as phone call prices fall and the number of potential new mobile phone users shrinks.
Along with music, some analysts expect video services to become one of the hottest advanced mobile phone features.
Cingular, a venture of SBC Communications and BellSouth, plans to introduce a video service using RealNetworks technology by year-end, the companies said.
Neither company disclosed details about the service, which will run over ain up to 20 markets by the end of the year.
As well as putting its media player software in Cingular phones, RealNetworks also plans to host video programming content for the operator's video service on its own computers.
Cingular also plans to use a mobile version of RealNetworks RealArcade video games, the companies said.