The move comes as Twitter's efforts to increase its user base and engagement have been sputtering, raising questions about its growth trend.
"Over the past decade, the tweet has evolved from a simple 140-character text message to a rich canvas for creative expression featuring photos, videos, hashtags, Vines, and more," said Twitter product manager Todd Sherman in a statement.
"So, you can already do a lot in a tweet, but we want you to be able to do even more. In the coming months we'll make changes to simplify tweets including what counts toward your 140 characters."
He added that updates would be implemented "over the coming months" and that developer partners would be given the tools to enable the change on sites that use Twitter.
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Relaxing the 140-character limit, which was set due to mobile phone text messaging constraints in place when Twitter launched in 2006, is expected to encourage more use and sharing of pictures, videos and links.
Sherman said a few other tweaks were being made to how Twitter works.
One new feature will enable users to retweet or "quote" their own messages "when you want to share a new reflection or feel like a really good one went unnoticed."
Twitter last month released a quarterly update that showed little change in its user base. In a key metric in the fast-evolving social media world, Twitter's trend was flat.
The number of monthly active users was 310 million, up three percent from a year ago and only slightly higher than the 305 million in the previous quarter.