The 43-year-old man, who has Brazilian and Spanish citizenship, was arrested at a Madrid hotel last week shortly after he arrived from the United States where he was hiding after fleeing Brazil in April, police said in a statement.
The suspect, identified only by his initials R.T.D., was a lawyer for Brazilian construction giant Odebrecht, the company at the heart of the pay-to-play scheme with Petrobras.
Odebrecht and other large Brazilian construction firms are suspected of overcharging Petrobras for work and passing on kickbacks to executives and politicians.
Police said the man is suspected of being the "financial brain" behind the Petrobras corruption scandal, which has shaken up Brazilian politics.
He is accused of money laundering, bribery and criminal association.
Brazil has issued an extradition order for the man. He must now appear before a judge in Spain who will decide if he can be sent back to Brazil, a police spokesman said.
Brazilian media identified the man as Rodrigo Tacla Duran and said he was arrested on Friday.
Police said it is the first arrest in Spain as part of the probe into corruption at Petrobras dubbed "Operation Carwash".
The operation was launched two years ago to tackle price fixing, bribery and political kickbacks at Petrobras.
It has led to dozens of convictions for bribery and money-laundering, and several top politicians are being investigated.
The "Carwash" probe, which has expanded to include other state firms and projects, has hobbled investment and fuelled a political crisis that led to the suspension of President Dilma Rousseff in May.