The offer had been 'leaked' online ahead of tomorrow's 99p sale but bosses have now confirmed it is true.
The offer is limited to one per person and will only last for 24 hours on November 5.
Users on HotUKDeals originally revealed the alleged 'leaked document' from the fast food giant saying it planned to test out a 99p Big Mac deal in November followed by free fries for one day only.
The document says McDonalds will sell Big Macs for 99p on Tuesday, November 5 via the app as part of a test ahead of new deals being launched in 2020.
It will then give away free fries via the app on November 22 in the second deal of the month.
The deals will be available for app users outside of breakfast hours, according to the website.