A total of 4.075 billion euros worth of 12-month treasury bills carried an average interest rate of -0.414 percent, the treasury said.
The remaining 425 million euros worth of six-month treasury bills fetched an average interest rate of -0.49 percent.
The auction took place in a context when Spain's risk premium stood at around 70 points while rating agencies, such as S&P, Moody's or Fitch, have recently upgraded Spain's sovereign debt rating.
The treasury will hold a treasury bonds auction on May 17 and another one on May 22 when it will place three- and nine-month treasury bills. (1 euro=1.19 U.S. dollars)