The move follows a competitive pitch that is understood to have also included Mindshare and Initiative among other international agencies. Dentsu has held the account since 2011.
Billings for the account are not yet known, but sources have indicated that the business was worth around US$40 million last year.
Representatives from Dentsu Media were not able to comment on the news.
Indosat has been operational since 1969. Mobile services are its principle business and it is understood to control a share of more than 20 per cent of that market.
