The social media platform normally limits posts to 280 characters.
The company also said newsletter firm Revue, which it bought last year, would now be part of Twitter Write along with the ‘Notes’ feature.
The tech giant said it was folding Revue into the new Notes product, which allows writers to embed gifs, photos and other features into long-form essays that can be read on and off Twitter.
The long-form feature is being tested by a small group of writers and Twitter did not give more details on its wider roll-out.
The company currently has a 280-character limit on tweets, a feature it began testing nearly five years ago. Several writers have already published Notes on the platform, which appear as long-form posts that can have tweets, videos, and images mixed in. The test will run for two months and involve a small group of writers in Canada, Ghana, the UK and US. The new feature aims to keep audiences in the Twitter eco-system, with readers able to see a headline and access the longer note by clicking on a link.
“Since the company’s earliest days, writers have depended on Twitter to share their work, get noticed, be read, create conversation - everything but the actual writing,” the company said, making the announcement using the new Notes product. “With Notes, the goal is to fill in that missing piece.”
At the moment, Twitter users have to write long-form content in the form of broken threads which may be jarring for people to read.
Users will also be able to edit their notes after they have been published.
The latest move comes amid scrutiny of the Twitter's business prospects, as Elon Musk's planned takeover of the company raises questions about its direction.
The firm announced in April it was working on an edit button, shortly after Musk – who had called for such a feature – revealed he had bought a large stake in the company. Twitter said the events were unrelated.