Direct proof of use
Russia deployed Su-25SM/UB attack aircraft to Hmeimim Air Base in Latakia before the start of its September 2015 air campaign in Syria. RAND identifies the initial Russian air wing as including twelve Su-25SM/UB strike aircraft assigned to close air support, while CNA's campaign analysis describes a squadron of twelve modernized Su-25SMs arriving at Khmeimim on September 19, 2015.
The aircraft were used by Russian forces on the Syrian government side. RAND describes Su-25 aircraft supporting regime operations in northwest Syria, and a Russian military district commander told TASS in March 2016 that returning Su-25 crews had flown more than 1,600 sorties and dropped about 6,000 bombs during the Syrian operation.
Sources: RAND Russian Air Campaign, CNA Russian Air Campaign Preliminary Analysis, TASS Su-25 Syria Sorties
Narrative
The Russian Su-25s fit the early Syrian air campaign's emphasis on tactical aircraft operating from Hmeimim in support of Assad's government. RAND characterizes the initial force package as a mix of Su-24 bombers, Su-25 close-air-support aircraft, Su-30SM fighters, and Su-34 fighter-bombers, and assesses that Russia's air effort was more effective against the Syrian opposition than against ISIS.
The aircraft's documented role was battlefield attack rather than transfer to Syrian control. The available sources describe Russian Su-25s flying strike and close-air-support missions for the pro-government campaign; they do not establish Syrian Air Force Su-25 operation as a separate possession or transfer claim.
A February 3, 2018 shootdown over Idlib provides a later dated marker of continued Russian Su-25 operations in the theater. The Defense Post reported that Russia's defense ministry confirmed a Russian Su-25 was downed while flying in the Idlib de-escalation zone, with preliminary information pointing to a possible man-portable air-defense missile.
Sources: RAND Russian Air Campaign, Defense Post Idlib Su-25 Shootdown