This represented an increase of 137,523 on a month earllier.
Movistar was the market leader, operationg 3.2 million lines, or 57.3% of the total.
All told, the number of FTTH lines increased by 1.7 million in the year to May 2017.
This was in contrast to DSL, where the number of lines fell by 1.4 million.
Meanwhile, the mumber of fixed broadband retail lines in Spain stood at 13.9 million in May, representing a ratio of 30 lines per 100 inhabitants, compared to 28.8 a year earlier.
Although masmovil showed the greatest growth, 93.7% of all lines were still concentrated in the hands of the three main operators: Movistar (41.8%), Orange (28.6%, including Jazztel) and Vodafone (23.3%, including Ono).