The pilot managed to eject and parachute to the ground before being captured. He was shot and killed when he resisted capture by opening fire from his pistol on the al-Qaeda-linked militants who tried to seize him alive.
The group Tahrir al-Sham released a statement on social media quoting a commander in charge of its air defences as saying one of its fighters had hit the jet during an air raid over the city of Saraqeb in the northwestern province of Idlib.
Tahrir al-Sham includes the group formerly known as the Nusra Front, which served as al Qaeda's Syrian branch
Russia's defence ministry confirmed the pilot had been shot down, saying: 'A Russian Su-25 aircraft crashed during a flight over the Idlib de-escalation zone.
'The pilot had enough time to announce he had ejected into the zone, under the control of al-Nusra Front fighters.
'The pilot was killed in fighting against terrorists.'
It added that 'according to preliminary reports, the plane was shot down by a portable anti-aircraft missile system.'
The ministry also said 30 militants had been killed in air strikes carried out in the Idlib area.
A video circulating on social media shows a lifeless body of a man, his face stained with blood, as bearded gunmen stand around him. One of the armed men shouts: 'He is Russian.'
The pilot was later named on social media as Major Roman Filipov.
Rami Abdel Rahman, head of The Syrian Observatory for Human Rights, said: 'The pilot was killed as he fought Islamist rebels who had shot down his plane and were taking him captive.' The Russian defence ministry confirmed one of its pilots has been killed.
Russia is a key ally of President Bashar Assad, and has been waging a military campaign on behalf of his forces since 2015.
Since then, Syrian government forces have captured wide parts of the country and in recent weeks have been marching in the rebel-held northwestern province of Idlib. The province is also a base for al-Qaida's branch in Syria and other Islamic groups.
The opposition's Aleppo Media Center said the plane was a Russian-made SU25 but did not say whether it was Russian.
Syrian troops launched a fierce offensive on Idlib in late December, with backing by Russian warplanes.
'There have been dozens of Russian air strikes in the area over the past 24 hours. This plane was also carrying out raids there,' said Abdel Rahman.
Opposition factions have shot Syrian regime planes in the past, but downing Russian warplanes is much rarer.
In August 2016, a Russian military helicopter was shot down over Syria and all five people on board were killed.
Moscow began conducting air strikes in Syria in September 2015. Two months later, Turkey shot down a Russian warplane.
It comes as a Syria peace congress hosted this week by Russia in the Black Sea resort of Sochi has been snubbed by Syria's main opposition and the Kurds.
It agreed on the creation of a commission to discuss the country's post-war constitution.
UN Syria envoy Staffan de Mistura, who attended the meeting, said the United Nations would lead efforts to form the commission.
A copy of the final statement did not mention the fate of Moscow's ally President Bashar al-Assad.
In a statement quoted by state news agency SANA, a source in Syria's foreign ministry said the talks in Sochi were 'the cornerstone of the political process and solid base upon which dialogue will be launched from now on'.
Around 1,400 delegates attended Tuesday's meeting, as part of a broader push by regime-backer Moscow to consolidate its influence in the Middle East.
But the main opposition group, the Syrian Negotiations Committee, boycotted the meeting as did representatives of Syria's Kurdish minority.
