The world has a right to know who carried out this "barbaric act", said Spanish Prime Minister Mariano Rajoy.
"The world has a right to know what has happened and everybody will have to do everything possible to make sure that a savage act like this doesn't happen again," he said after a cabinet meeting, also attended by Spain's new king Felipe.
Rajoy's words came after Spain's foreign ministry expressed its condolences and said it shared the pain of families and loved ones of the 298 victims.
"Spain hopes an investigation can explain the circumstances of the accident as quickly as possible," the foreign ministry said in a statement.
The ministry also urged the parties involved to participate fully in the investigation.
Spain's national flagship carrier Iberia also tweeted its support for the victims and for staff at Malaysian Airlines, a fellow member of the oneworld air alliance.
Barcelona-based Vueling airlines has said it will divert flights from the area where the plane crashed.
The Boeing 777, travelling from Amsterdam Schiphol to Kuala Lumpur, was blown out of the sky at 10,000 metres, by what US officials claim was a Russian-made surface to air missile.
Russia's President Vladimir Putin has blamed the Ukrainian government, saying the tragedy would not have happened if it had not escalated the war in eastern Ukraine, while the Kiev government blamed the pro-Russian rebels for shooting down the plane.
Ukraine's President Petro Poroshenko described the crash of flight MH17 as a "terrorist act".
US vice president Joe Biden, meanwhile, described the plane as being "blown out of the sky".
The plane was also carrying 154 Dutch citizens, 27 Australians, 23 Malaysians, 11 Indonesians, nine Britons, four Belgians, three Filipinos and a Canadian.