Private individuals will no longer be allowed to advertise guns on Facebook and Instagram, the photo-sharing service owned by Facebook.
Facebook had already banned the sale of guns without identity checks, but the new rules aim to stop all gun trade between individuals on the sites.
Businesses can still advertise guns on Facebook and Instagram.
The move comes three weeks after US President Barack Obama unveiled new restrictions on gun purchases.
Mr Obama's executive actions included background checks for all gun sellers and the requirement that states provide information on people disqualified from buying guns due to mental illness or domestic violence.
The rule change brings gun sales under the same restrictions placed by Facebook on illegal drugs and pharmaceuticals by Facebook. The site has 1.59bn users worldwide.
Facebook "was unfortunately and unwittingly serving as an online platform for dangerous people to get guns", Shannon Watts, of the Everytown for Gun Safety campaign group, told Associated Press.
The group said it had found evidence that guns had been bought on the site and used to kill others in two cases.
Everytown for Gun Safety was one of a number of groups that had called on Facebook to change its policy.
The National Rifle Association (NRA), which opposes changes to gun legislation, has not yet responded to Facebook's decision.
In 2014, the NRA said previous moves by the network to limit gun advertising were insignificant.
In late 2013, New York's attorney general, Eric T Schneiderman, wrote to Facebook, alerting them to "a number of groups in which users promoted the sale of assault rifles, handguns, rifles, shotguns and gun parts".
Several hours after Facebook's announcement on Friday, dozens of groups on the site advertising private gun sales remained live.
Some users writing on the groups' walls suggested starting new groups under inconspicuous names to avoid detection.