The creative pitch took place on 16 May, and the agency pitched against Dentsu Taiwan, JWT, and McCann Worldgroup, according to sources. Those agencies did not respond to a request for confirmation in time for publication.
The messaging-app company announced the pitch result on 19 May.
Line Cookie Run launched globally earlier this year. The game resembles Mario Brothers, with a gingerbread cookie trying to avoid obstacles while on the run.
Line previously ran a global TVC for the game in Taiwan. Dentsu K is understood to be discussing its creative plan with the client.
This is first mobile account for the Dentsu K team. Its clients include Uniqlo; GU (a sub-brand of Uniqlo); 7-Eleven, Visit Japan Campaign; Panasonic's Viera TV, beauty electronics, and Lumix cameras; and Honda Automobile.
The agency rebranded in August 2013.