The German's best time of one minute 23.951 seconds was 0.138 quicker than his Finnish team mate and half a second faster than runaway championship leader Nico Rosberg in the Mercedes.
Ferrari were the only ones using the quicker, soft tyres however, having opted for more sets than their rivals as they feel their car is better suited to them.
Rosberg, who was on pole in 2015 with a time of 1:24.681, is chasing his fifth win in five races this year and his eighth in a row.
His triple world champion team mate Lewis Hamilton, 43 points behind Rosberg in the standings, was fourth fastest in the session with the Mercedes drivers lapping on the slower medium tyres.
The Briton has been hit by power unit failures in the last two races, starting from the back of the grid in China and 10th in Russia, and Mercedes said they had changed the turbo and a part of the energy recovery system for Barcelona.
But they had just one example available due to the limited time to change the design between the last race in Russia and Spain and getting it ready in time had taken a "superhuman" effort.
The two Red Bull drivers were fifth and sixth, with Dutch teenager Max Verstappen making his practice debut with the energy drink company's senior team after leaving Toro Rosso in a swap with Russian Daniil Kvyat.
Ricciardo lapped in 1:25.416 with Verstappen's best time being 1:25.585, despite the teenager having some early problems with the car's acceleration.
Kvyat was 12th, with Spanish team mate Carlos Sainz eighth.
Spain's twice champion Fernando Alonso was 10th for McLaren.
Mexican Sergio Perez had a difficult morning, stopping with two thirds of the session remaining after smoke came from the rear of his Force India car. The team have brought major aerodynamic upgrades to Spain.
French rising star and reserve Sebastien Ocon made his practice debut with Renault, taking the car that Britain's Jolyon Palmer will race on Sunday, but was unable to set a lap time due to technical problems.
(Reporting by Alan Baldwin, editing by Ken Ferris)
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