The stake was sold at 10.2 euros per share, which represented a discount of 5.7% from Acerinox's Thursday closing price.
Acerinox were down 5.8% on Friday morning.
After the placement, Nippon Steel still held a 7.9% stake in the company.
The company said 10 months ago that its stake in Acerinox was "financial", that is, susceptible to being sold, according to local newspaper Cinco Dias.
(Reporting by Cristina Galán and Tomas Cobos; editing by Inti Landauro and Jason Neely)