The social media platform normally limits posts to 280 characters.
Twitter said the move was a response to seeing people use the platform to post pictures of longer announcements and steer followers to outside newsletters.
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."
The move follows Twitter's purchase last year of Revue, a Dutch newsletter start-up.
On Wednesday, it 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.
Users will also be able to edit their notes after they have been published.
Social media expert Dr Laura Toogood said the trial was a significant step for Twitter. She said the feature would encourage people to stay within the platform itself, rather than linking out to other websites that can host long-form content.
"Adding this extra capability means that Twitter is now in a position to compete with some of the popular blogging platforms and potentially attract a new audience and a different type of user," she told the BBC.
"It will also encourage existing users to blog within Twitter, rather than move to other websites for this purpose, which will help to retain their audience."
In 2017 Twitter increased the maximum number of characters for tweets from 140 to 280, following a trial among a small group of users.
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.