Apple has made its first move into the already crowded wearable computing space, taking the wraps off Apple Watch, a smart wristwatch that CEO Tim Cook breathlessly declared on a stage in San Francisco will “redefine what people expect from this category”.

The watch — Cook’s “one last thing” — came during one of the busiest keynotes from Apple in years, in which it took the wraps off two larger-screen iPhones and unveiled a new smartphone payments platform called Apple Pay.

Cook called the watch the next chapter in Apple’s history, describing it as a “comprehensive health and fitness device” and an “extremely precise and customisable timepiece”.

Apple says it has spent years developing and refining the watch. Key to its design is a “digital crown” that allows the wearer to navigate the user interface without a finger covering the display.

The Watch will be available in two sizes with six straps. There are also three different finishes, the basic unit, a sports model and one which is made with 18-carat gold.

It integrates with Siri, Apple’s voice-driven software assistant. However, the watch doesn’t work without a companion iPhone.

Users can choose from a range of customisable watch-face designs. It has a built-in heart-rate monitor and gyroscope. It features inductive charging, too.

The Watch will start at a recommended retail price of US$349 and will go on sale in the US in early 2015. It’s not immediately clear when the device will go on sale in South Africa.