Duolingo was launched in 2011 and now has more than 90 languages to learn on iOS, Android and its website.
The Scottish Gaelic course was launched in the run-up to St Andrew's Day last week, with nearly 20,000 people signed up ahead of its release.
It has now accrued has more than 65,000 learners in just five days, developers said.
According to the last census in 2011, 57,375 people could speak Scottish Gaelic and the University of Edinburgh has suggested 5,460 are currently studying on other platforms - 62,835 in total.
Colin Watkins, Duolingo's country manager in the UK, said: "To have more people learning on Duolingo in under a week than can speak the language or are currently learning elsewhere is amazing.
"It's testament to how easy, fun and effective learning on Duolingo can be whether it's learning Scottish Gaelic or Spanish, Latin or Japanese.
"We've seen everyone from Alex Massie, Sanjeev Kholi, Irn-Bru and the author of the Outlander get involved.
"We hope people from across Scotland sign up and start learning. At the very least it will help you understand place names and road signs."
He added there is "still some way to go to catch Esperanto, Navajo and Klingon", which have around 300,000 users each.
Users learn a language by playing various games including typing words or phrases, matching grammar and also speaking or listening to sentences to earn "crowns" and progress their knowledge through a number of levels.
Words and phrases in early rounds of learning Scottish Gaelic include boy and girl, various animals, and discussing chicken and Irn-Bru.
Ahead of the course's launch, Deputy First Minister John Swinney said: "The Gaelic language is a vital part of Scotland's cultural identity and we want to ensure those who wish to learn and use the language are given every opportunity to do so.
"I therefore warmly welcome the launch of the new online resource from Duolingo which is another useful tool to promote the use of Gaelic.
"This sits alongside the recently updated Learn Gaelic website, which contains a wealth of adult learning materials for those with an interest in the language."