The Spanish public prosecutor's office said Tuesday it had opened a probe into possible "serious violations of international humanitarian law by Russia in Ukraine".
The aim is to "determine the criminal nature" of Russia's invasion of Ukraine, the office said in a statement, just hours after Germany's federal prosecutors opened a similar probe.
"The aggression suffered by the sovereign nation of Ukraine is an unjustified act of war that is not covered by any international regulations," the Spanish prosecutor's office said.
It has resulted in "serious violations of international humanitarian law," it added.
The chief prosecutor of the International Criminal Court in The Hague announced last week that he was going ahead with an investigation into alleged war crimes and crimes against humanity committed in Ukraine since Moscow's invasion on February 24.
Earlier on Tuesday, Germany's federal prosecutor said it had opened a probe into suspected war crimes by Russian troops since the invasion.
Like Germany, Spain has repeatedly prosecuted atrocities committed abroad using the legal principle of universal jurisdiction.
This doctrine allows countries to try people for crimes of exceptional gravity, including war crimes and genocide, even if they were committed in a different country.
Russia has come under intense criticism for its assault on Ukrainian cities, in operations that Kyiv and Western governments say have included attacks on schools, hospitals and residential blocks.
US Secretary of State Antony Blinken said on Sunday that Washington has seen "very credible reports" that Russia has committed war crimes during its invasion of Ukraine.
Russia calls its actions - which have triggered the worst refugee crisis in Europe since World War Two - a "special military operation".
More than two million people have now fled Ukraine since Russia launched its invasion, according to the latest data from the United Nations Refugee Agency.