Users can paste a link of any length into the message composition box on the site, according to a company blog post.
"After you've composed your Tweet and you hit the "Tweet" button, we'll shorten the link so that it only takes up 19 characters," it said.
"You no longer have to worry about fitting a long link into the 140 character limit," Twitter said on its support page.
Users currently rely on third-party link-shortening services, which will continue to be supported on Twitter.
The service is being rolled out by Twitter to a small percentage of users to start with. After 13 characters of a URL are entered, a message appears to let the user know that the link will be shortened. Even after the message character limit is reached, users can continue to add text to the URL, Twitter said.
Once the message is posted, it will be assigned a t.co link ID, but the link will appear as a shortened version of the original URL, so that people who see the message will know the site they are going to.
Twitter's link service is only used on links posted on Twitter and is not available as a general shortening service. The company emphasized the security aspects of its new service. URLs converted by Twitter's link service are checked against a list of potentially dangerous sites, and when there's a match, users will be warned before they continue, it said.
Twitter also added language support for Brazilian Portuguese to its site on Tuesday. The site already supports nine languages, including French, German, Spanish, Korean, Russian, and English. Twitter is ranked 13 among the top web sites in Brazil, according to web traffic monitoring service Alexa. It trails Google, Orkut, and Facebook in that country. The rank is calculated using a combination of average daily visitors and page views over the past month.