The app, which intends to raise awareness of youth homelessness and reach out to a younger audience, claimed the top spot in the iTunes chart on 15 May following surrounding its aims and methodology.
More than 100,000 iHobo apps have been downloaded from the UK store to date.
The app places decisions about a fictional homeless person's life in the hands of iPhone users - including offers of food, money and emotional support.
The app uses Apple’s most recent Push Notification technology to send alerts to the user when the iHobo character requests help.
Paul Marriott, chief executive of Depaul UK, said: "Depaul has always prided itself on taking risks on behalf of the young people we serve, and the development of iHobo was certainly a risk.
"It is deliberately designed to provoke conversations about youth homelessness and so its name and content had to be able to cause a reaction.
"For an organisation with no real marketing capacity, the opportunity to raise the profile of our cause and our charity to such a wide audience is immensely valuable."
Neil Simpson, Publicis London chief executive, called iHobo "modern charity marketing" which had successfully created greater awareness, engagement and return at a relatively low cost.
Publicis is working with Depaul UK on a pro bono basis.
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