The update is named Big Sur and numbered MacOS 11, marking the first major numerical revision since it released MacOS X almost 20 years ago.
It brings with it a complete overhaul of the operating system, with an entirely redesigned look that brings it more in line with the iPhone and iPad operating system, iOS.
That comes ahead of a similar transition in the hardware used for Apple's Macs, which later this year will start using Apple's own chips, rather than those made by Intel.
Away from the new look, the update brings new features including what Apple called the "biggest Safari update ever" and new features in the Maps and Messages apps.
But the significant nature of the update means that the early beta could have more bugs or small problems than usual. As such, users are warned only to install the update on devices that have been fully backed up, and Apple's official page warns that beta software should only be used on "non-production devices that are not business critical".
Apple also advises that users could install the new software either onto a "secondary system or device, or on a secondary partition on your Mac".
The full update will not be publicly available until "this fall", Apple has said.
Users can sign up to Apple's public beta programme by accessing the devoted page on its website. It is done by opting in and then downloading an update, and further versions of Big Sur should then be downloaded automatically.
Information about the public beta versions of iOS, iPadOS, tvOS and watchOS can all be found on that page, too.