The Russian military's reconnaissance Il-20 was shot down by the Syrian government missile defense systems responding to an Israeli airstrike. All 15 people on board were killed. Russia laid the blame on Israel, saying Israeli fighter jets had pushed the plane into Syria's line of fire, AP reported.
Syria's skies, where regional and international powers back different parties in the conflict, are increasingly crowded.
Shortly before the downing, Israeli strikes had hit targets inside Syria.
Russia launched a campaign in Syria to support President Bashar Assad in 2015, and though the involvement turned the tide of war in favor of Syrian government forces, Moscow has tried to play a careful balancing act.
President Vladimir Putin initially struck a reconciliatory note, blaming the downing on a "chain of tragic, fatal circumstances." But the Russian military came out on Sunday, renewing the accusations against Israel.
Russian officials said Syria's outdated S-200 systems weren't sophisticated enough to identify the Russian plane as a friendly one.
Monday's statement from Russian Defense Minister Sergei Shoigu said Russia will send the S-300 missile defense systems to Damascus within the next two weeks. Earlier in the war, Russia suspended a supply of S-300s, which Israel feared Syria could use against it.
Shoigu said Russia is now going to go ahead with the shipment because "the situation has changed, and it's not our fault." He also said that Russia would start to electronically jam aircraft flying in to attack targets in Syria.
"We are convinced that these measures will calm down some hotheads and keep them from careless actions which pose a threat to our troops," Shoigu said.
Russian Deputy Foreign Minister Mikhail Bogdanov told Russian news agencies that supplying S-300 to Syria is Russia's "own right".
The Kremlin said Russia's decision was not targeted against anyone and only serves to protect Russian troops in Syria. Spokesman Dmitry Peskov told reporters that recent findings by the Russian military showed an Israeli jet "deliberately" pushed the Russian Il-20 into the line of fire, enabling its downing.
Russia's stepped-up role in Syria enabled Assad's forces, which had been losing ground to terrorists, to gain the upper hand and reclaim significant territory held by them.
In recent months, the government recaptured many areas that were controlled by terrorists.
The downing of the plane came just hours after Russia announced it had reached a deal with Turkey to avert a Russian-backed Syrian government offensive against the northwestern province of Idlib, one of the last areas still in terrorist hands.
Shortly after Moscow's announcement, the Syrian president's office said Assad received a call from Putin and that the two discussed the latest developments, including the Idlib deal and the delivery of S-300s.
According to the statement, Putin reiterated that Russia holds Israel responsible for the downing of the plane. The Russian president also informed Assad of the S-300 delivery, it said. Assad expressed his condolences for the deaths of the Russian airmen, saying they were "carrying out noble mission, fighting terrorism in Syria."
The Kremlin said the two leaders discussed working toward "achieving a lasting normalization in Syria and restoration of its sovereignty, unity and territorial integrity."