Twitter currently limits tweets to 140 characters, but has doubled that to 280 characters for a small group of users.
The current character limit was "a major cause of frustration" for some users, the firm said in a blog post.
The firm has been suffering from slowing growth and the shift could be one way for the firm to widen its appeal and attract new users.
"Trying to cram your thoughts into a Tweet - we've all been there, and it's a pain," Twitter product manager Aliza Rosen wrote.
Ms Rosen said the longer character limit was being tested in all languages except Japanese, Chinese and Korean, which she said could convey more information in a single character.
"We understand since many of you have been Tweeting for years, there may be an emotional attachment to 140 characters - we felt it, too.
"But we tried this, saw the power of what it will do, and fell in love with this new, still brief, constraint," she said.
Twitter's founder Jack Dorsey was one of the first to try the new limit out, saying it was "a small change, but a big move for us".
Not all users were convinced. Writer Sarah Kendzior said the limit was too long.
Twitter, however, said the change could mean people tweeting more frequently.
"When people don't have to cram their thoughts into 140 characters and actually have some to spare, we see more people tweeting," Ms Rosen said.