The 6s has a better camera, faster processor and pressure-sensitive screen
Apple sold more than 13 million iPhone 6s and 6s Plus smartphones in the first three days of sale - a new record for the technology giant.
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Queues formed outside Apple Stores around the world when the phones went on sale on Friday.
Apple sold 10 million 6 and 6 Plus smartphones when the models went on sale last year.
The 6s has a better camera, faster processor and a pressure-sensitive screen for the first time.
Reacting to the figures, Apple chief executive Tim Cook said: "Sales for iPhone 6s and iPhone 6s Plus have been phenomenal, blowing past any previous first weekend sales results in Apple's history.
"Customers' feedback is incredible and they love 3D Touch and Live Photos, and we can't wait to bring iPhone 6s and iPhone 6s Plus to customers in even more countries on October 9."
After the launch of the iPhone 6 last year, Apple went on to sell more than 70 million units in three months.
The firm also posted the largest ever quarterly profit by a public company of £11.8bn.
There have been suggestions that - given the high sales figures last year and the modest upgrades to the latest device - demand for the 6s could drop.
However, the addition of China as a launch day country for the first time is likely to have boosted demand.
The new iPhones will launch in 40 more countries on 9 October, and will be available in 130 countries by the end of the year.