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 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.
In April, Twitter surprised users by saying it would in the coming months begin testing a new edit feature, which it called "the most requested feature for many years.
The company is in the middle of a $44 billion buyout by Tesla Inc CEO Elon Musk, the world's richest person according to Forbes magazine.