Across mobile phones in the US during August, Instagram received an average of 7.3 million daily visitors, while Twitter attracted 6.9 million users, a new report by ComScore said.
It also found users spent more time on the photo-sharing site than those who visited Twitter.
Instagram visitors spent an average of 257 minutes on the site via mobile phones in August, while those logging on to Twitter spent an average of 170 minutes viewing the site.
The news is likely to be warmly received Facebook, which increasingly competing with twitter for the lucrative mobile market.
Mark Zuckerberg's firm recently bought the social network for $715 million, sparking accusations of an 'app bubble' in Silicon Valley.
It is the first time that Instagram has pulled ahead of rival Twitter.
Mike Isaac of AllThingsD said the deal is a big step for Instagram.
'For a number of reasons, this is a pretty big deal,' he wrote.
'That the barely two-year-old Instagram could rocket up in user engagement and retention in such a short amount of time, eventually surpassing Twitter in the process, speaks to the sheer momentum of the photo-sharing product.'
However, the number of unique visitors to Twitter across the whole of August was higher than for Instagram, with Twitter recording 29 million unique visitors to the site, compared to Instagram who received almost 22 million.
Earlier this month Facebook finalised the purchase of Instagram, according to the photo-sharing website's founder.
The social networking site bid $1bn (£628m) for Instagram in April, and was given the green light for the deal from US regulators at the end of August.
But, after the sharp fall in Facebook shares since the firm went public in May, the cash plus share offer ended up being worth nearer $700m.
'Very excited to announce that we're now officially joining the Facebook family," Instagram founder Kevin Systrom wrote in a Facebook post at the time.
'Instagram has continued to grow and succeed beyond what we ever imagined, and we're so proud of all the team has accomplished.
'I'm psyched for the next chapter of this long journey.
'I want to personally thank everyone in the Instagram community for the last two years of inspiration.'