The photo-sharing service's popularity surged following its $1bn (£629m) acquisition by social networking giant Facebook earlier this week.
Instagram, which is available as a free download, beat popular apps such as Draw Something and Angry Birds Space to the top spot to add to a user base of more than 30 million.
Facebook CEO Mark Zuckerberg has pledged to keep the smartphone application running as a separate brand, but reports suggest that the buyout has convinced some users to jump ship due to concerns over the social networking site's privacy policies.
Scores of subscribers took to Twitter to express fears that Facebook may introduce adverts to the service or remove photographs that violate the terms of its user agreement.
Instagram was released for Android devices earlier this month, attracting more than one million downloads in its first day.
> Facebook's Instagram deal ignites fears of new tech bubble