CEO Steve Ballmer made the annoucement in a keynote that kicked off Microsoft's SharePoint Conference 2009 in Las Vegas.
When pressed for details, a Microsoft spokeswoman said the company did not have a specific timeline beyond Ballmer's pinning the beta to next month.
So far, Microsoft has offered a preview of its next desktop suite only to a relatively small group of testers. It has also opened the online edition , Office Web Apps, to a similar preview.
Office Web Apps includes lightweight versions of Word, Excel and PowerPoint and will be made available to millions free of charge in the first half of next year, the only timetable Microsoft has set for Office 2010's ship date.
Anyone will be eligible to test drive the Office 2010 beta, said Microsoft today. However, the company declined to answer questions about whether the number of copies of the beta will be limited - as it tried to do with the Windows 7 beta earlier this year - or be available only for a limited time, as was the Windows 7 release candidate.
Last summer, Microsoft said that it expected to distribute millions of copies of the Office 2010 public beta.
In April, Microsoft said that it would not offer users the chance to test Office 2010, as it had done with other editions, including Office 2007. The company quickly backtracked, saying that it had simply given "the wrong impression" about its plans.
Also unknown is whether Microsoft will charge users to download the beta, a tactic it used with the second beta of Office 2007, when it let customers try out the suite from within their browsers for free, but charged them $1.50 (£1) to download the preview.
The latter move, Microsoft said in July 2006, was because "the beta 2 downloads have exceeded our goals", prompting it to "implement a cost-recovery measure".
Microsoft may use a new technology, called Click-To-Run, that debuted in July with the Technical Preview, to deliver the beta of Office 2010.
Click-To-Run "streams" pieces of the suite to users who begin a download, letting them start using the suite within minutes. While users work with the suite, the remainder of the code is downloaded in the background.
Two weeks ago, Microsoft said it would use Click-To-Run to offer a limited-time trial of Office 2010 when the final bits ship next year.
The company will also offer an advertising-supported version of Office 2010 to computer makers, who will install it on their new PCs as an alternative to the retired Microsoft Works. Dubbed Office Starter 2010, it will include scaled-back editions of Word 2010 and Excel 2010.
An after-market 'key', purchased either on a card at electronics retailers or online from Microsoft, will unlock the appropriate for-a-fee version, so that no additional software need be downloaded. Customers will be able to upgrade from Starter to Office 2010 Home & Student, Home & Business or Professional. Microsoft has not yet announced prices for Office 2010.
Ballmer made the Office 2010 beta announcement at the same time he revealed some of the features of the upcoming enterprise SharePoint 2010 software. He said that a public beta of SharePoint 2010 would also be available to the public next month.