It means the Apple Watch Series 3 can receive phone calls, access internet services and stream music without being linked to an iPhone.
Apple recently overtook Fitbit to become the world's joint-top wearable tech-maker alongside Xiaomi, according to one study.
The firm will later unveil new iPhones including a high-end X model, if an earlier leak proves accurate.
Other companies - including LG and Samung - have previously sold smartwatches with in-built cellular capabilities, but battery-life restrictions and other issues limited interest.
"Apple's ability in the past to generate new markets when others thought they were dead is legendary," commented Neil Mawston from the consultancy Strategy Analytics.
"For people like joggers, runners and cyclists who possibly want to do hardcore sports outdoors without carrying two devices, an LTE Apple Watch could be something of a blessing."
The latest version of the Watch's operating system - which will also be available to earlier models - will include new heart monitor functions.
It will warn owners if their heart rate becomes elevated when they are not active or if its rhythm becomes irregular, to flag the possibility of disease.
The 4G Apple Watch will cost $399 (£300) and be released on 22 September.
Apple's previous flagship phones - the iPhone 7 and iPhone 7 Plus - caused controversy by abandoning a traditional headphone socket.
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