Russian Ambassador to Turkey Andrey Karlov died after being shot by an unidentified gunman in Ankara late on Monday, the embassy announced.
Karlov was shot while making a speech at a painting exhibition in the Turkish capital, the Russian embassy said, adding it believed that it was a radical Islamist attack.
Karlov was envoy since July 2013.
The attack comes a day before Turkish Foreign Minister Mevlut Cavusoglu heads to Moscow for talks on Syria with his Russian and Iranian counterparts.
CNN reported the attacker shouted "Allahu Akbar" before firing at the diplomat.
Russian President Vladimir Putin has said the killing of the Russian Ambassador to Ankara is designed to spoil the Russia-Turkey ties.
Putin said Andrey Karlov's killing in the Turkish capital was also intended to derail Moscow's attempts to find, with Iran and Turkey, a solution for the Syria crisis, the Independent reported on Monday.
He said Russian investigators have opened a criminal case into the killing.
In televised comments, Putin, speaking at a special meeting in the Kremlin, ordered security at Russian embassies around the world to be stepped up and said he wanted to know who had "directed" the gunman's hand.
He heaped praise on the murdered Russian ambassador, who was shot at as he gave a speech at an Ankara art gallery, and made clear that Moscow's response to his assassination would be robust.
"A crime has been committed and it was without doubt a provocation aimed at spoiling the normalisation of Russo-Turkish relations and spoiling the Syrian peace process which is being actively pushed by Russia, Turkey, Iran and others," said a stern-faced Putin.
"There can only be one response - stepping up the fight against terrorism. The bandits will feel this happening."
Putin, who said he personally knew the slain envoy, said he had agreed in a phone call with his Turkish counterpart President Recep Tayyip Erdogan that Russian investigators would soon fly to Ankara to help the Turks with the investigation.
"We must know who directed the killer's hand," Putin told Foreign Minister Sergei Lavrov, Sergei Naryshkin, the head of his SVR foreign intelligence service, and Alexander Bortnikov, the head of the domestic FSB security service.
Putin ordered security at Turkish diplomatic facilities in Russia to be stepped up and said he wanted guarantees from Turkey about the safety of Russian diplomatic facilities.
"I also ask you to implement the agreed proposals on strengthening security at Russian diplomatic facilities abroad," Putin told the meeting.
Meanwhile, the United States also condemned the incident.
"This heinous attack on a member of the diplomatic corps is unacceptable, and we stand united with Russia and Turkey in our determination to confront terrorism in all of its forms," said US National Security Council Spokesperson Ned Price in a statement.
UN Secretary-General Ban Ki-moon called it as a "senseless" act of terror.
"The Secretary-General is appalled by this senseless act of terror and emphasizes that there can be no justification for the targeting of diplomatic personnel and civilians," said a statement released by Ban's spokesperson.
"The Secretary-General is following the unfolding situation closely and wishes the other people who were reportedly injured in the attack a speedy and full recovery," it added.
"Don't forget Aleppo! Don't forget Syria!": Turkish gunman reportedly heard shouting after shooting dead Russian ambassador in Ankara pic.twitter.com/0mkz5huqtiâ€” Middle East Eye (@MiddleEastEye) December 19, 2016