The switch from 140 characters to 280 characters is currently just an experiment and, as such, the majority of users are still stuck on the old limit.
The social networking giant believes the current 140-character limit means some users find it hard to express themselves in languages such as English and Spanish.
Just 0.4% of tweets sent in Japanese have 140 characters, compared to 9% of English tweets, the company said.
This is due to languages like Japanese and Chinese being able to convey more in one character, it added.
The new feature will be available in all languages except Chinese, Korean and Japanese.
Can’t fit your Tweet into 140 characters?
We’re trying something new with a small group, and increasing the character limit to 280! Excited about the possibilities? Read our blog to find out how it all adds up. https://t.co/C6hjsB9nbL
— Twitter (@Twitter) September 26, 2017
The trial is being rolled out to a small percentage of Twitter’s 328 million monthly users.
Twitter’s product manager Aliza Rosen said: “We want every person around the world to easily express themselves on Twitter, so we’re doing something new: we’re going to try out a longer limit, 280 characters, in languages impacted by cramming.”
Rosen stressed that Twitter was still about brevity, adding: “It’s what makes it such a great way to see what’s happening. Tweets get right to the point with the information or thoughts that matter. That is something we will never change.”
Earlier this year, the social media site removed character limits from Direct Messages sent privately between users.
It also stopped counting images and other media attached to tweets as part of the character limit in an attempt to improve the service.
Twitter revealed in its Q2 earnings this year that revenue fell 5% on the same quarter last year to $574m (£436m).