The partnership is the largest of the 21 search deals Yahoo has announced this year with mobile phone operators, the Sunnyvale, California company said.
Mexico City-based America Movil has 143 million wireless subscribers.
Yahoo is racing to attract subscribers to Internet services delivered via mobile phones, rather than computer browsers. It launched oneSearch in the United States in January 2007.
The service lets users search the Web on the first screen they call up, unlike browsers designed for computer users, which force phone users to navigate through several screens.