The foundation announced last year it would start running three rounds of its News Challenge each year, which funds "new ways to digitally inform communities" with awards of up to $5 million a year.
The first round of the challenge, which centred on networks, announced its six winners earlier this year. The second challenge, for data, will announce its winners next month.
Applications can now be submitted for the mobile challenge until noon EDT (5pm BST) on 10 September.
Winners of the third round will be revealed early next year.
According to an announcement by the Knight Foundation, applicants will be asked eight questions on their "idea on using mobile to improve news, information, democracy and communities, and your ability to execute on it".
"We also encourage you to link to anything that helps you to make that case."
In an announcement earlier this month, introducing the topic, the foundation said its wants "to learn about new approaches for using mobile to inform and engage communities, and build the foundation for others to do more in the future."