When tapped, the downvote button hides a comment, and gives users additional reporting options like "Offensive", "Misleading", and "Off Topic".
The move has always been anticipated - and demanded by users - but the social network says it is not a "dislike" button.
Facebook is testing a Reddit-style downvote button for comments on public Page posts, a brand-new mechanism and a major change in how Facebook grades content. The button is being tested by a small number of U.S. users only.
The company said, "The intention is to give users a way to flag inappropriate, uncivil or misleading comments".
Christina Hudler, who lives in Jacksonville, North Carolina, was among the Facebook users who noticed the mysterious feature pop up on Thursday.
Facebook has long faced an issue over how best to let people express less-than-positive feelings about people's posts and comments. The new feature is a downvote button that lives right underneath comments made on posts on the network that falls in line between the reply and thumbs up counter on the comments.
Facebook said there are now no plans to expand the feature.
The site says the number of downvotes will not be visible and only appear on Page posts, and not posts on Groups or public figures.
"That doesn't seem like the kind of community we want to create", Zuckerberg said back then.
Taylor Lorenz, a reporter for The Daily Beast, tweeted screenshots of the feature on 8 February.
While this feature is making headlines, some people have linked this with Reddit's famous "Downvote" button. We will keep you updated as and when we get any information from Facebook.
User-demand for a "dislike button has been high ever since the introduction of the 'like" button, with users complaining that they should be able to voice whether they do not agree with a particular post.
Speaking at a town hall event in September 2015, Zuckerberg said he didn't want Facebook to turn into a place where people "upvoted" and "downvoted" comments.