Sources told Turkish media that the attacker detonated an explosive vest on the bus at the entrance to a camp for displaced people late on Sunday.
The death toll was likely to rise, activists monitoring the conflict said.
A news agency linked to the jihadist group Islamic State reported that one of its militants was behind the attack.
Syrian opposition Orient TV reported that those killed were from “numerous” rebel factions, all of which are opposed to IS.
Photographs posted on social media after the bombing showed the burnt-out remains of a bus and medics treating badly injured men.
Large parts of Syria’s north-western province of Idlib, where Atmeh is located, is controlled by rebel factions supported by the Turkish government.
Saudi-owned Al-Arabiya TV reported that the fighters targeted on Sunday were part of a 600-strong force that had been preparing to advance towards the countryside north of the divided city of Aleppo, 50km (30 miles) east of Atmeh.
On Monday, a Syrian military official and activists said government forces had repelled a rebel assault south-west of Aleppo, forcing rebel fighters to retreat from positions near a cement factory that they had seized the previous day.
The UK-based Syrian Observatory, which monitors the five-year-old conflict in Syria through a network of sources, said at least 35 rebels had been killed in the battle. Seventeen died on the government side, it added.
The fighting in Aleppo, once Syria’s commercial and industrial hub, has escalated in the past week, with the rebels severing the main route into the government-controlled west of the city. The offensive sought to break a siege by pro-government forces, who encircled the rebel-held east in July. -BBC