Earlier, the airline said flight disruption through the strike was "minimal" and only affected three percent of its Spanish flights.
The employees, who are demanding better working conditions, began an initial three-day strike on June 24 during which 129 Spanish flights were cancelled, the union said.
Ryanair is the airline that transports the most passengers in the Spanish market, offering more than 650 routes to 27 airports, the company says.
The initial strike in Spain coincided with industrial action by the airline's staff in Portugal, Belgium, Italy and France demanding respect for labour rights and higher wages.