The Apple Watch, which is controlled using the crown, will be available early next year and will cost $349 (£217).
The watch, which needs an iPhone to work properly, has sensors on the back to detect pulse rate and can track your physical activity.
It charges via a magnetic connector which snaps on to the back. The device comes in two sizes and has multiple straps plus other personalisation options.
The screen resolution on the Plus version will be sharper than previous iPhones, at 401 pixels per inch rather than 326.
Apple is taking advantage of the larger screen by offering a horizontal view of the home screen. Usually, icons are stacked vertically.
Apps that are not optimised for the larger screens will be automatically scaled up.
Apple also revealed a new payments system, called Apple Pay, which can be used by the new iPhones and the Apple Watch. It enables you to purchase items via contactless payments.
The new iPhones will come with iOS8, the latest operating system for Apple devices, pre-installed. It includes an updated iMessage app plus more health-related features.
iOS8 will be available to download for other Apple devices from September 17.