Dockers will strike on alternate hours for 48-hour periods between June 26 and 28, June 29 and July 1, July 3 and 5 and July 6 and 8, the UGT worker union said.
Spanish port terminals have already been hit by major disruptions as container shipping companies re-routed their cargos to avoid strikes in the country, opting for Portugal, Malta or Morocco instead.
Dockers are angry over reform of port labour practices passed by Spanish lawmakers in mid-May, which aimed to crack down on closed-shop hiring in a heavily unionised sector.
Subsequent negotiations over how to implement the new rules between employers and workers broke down in a dispute over how to safeguard 6,000 jobs.
The strikes had so far caused losses of 110 million euro ($123 million) to Spain's economy, the public works ministry said in a statement adding that this was due to business being diverted elsewhere.
The southern Spanish port of Algeciras is particularly vulnerable to the strikes as much of the goods traffic that passes through it is not ultimately destined for Spain. It is used by companies transferring cargos from one boat to another.
Exports have helped underpin Spain's economic recovery over the past four years. Some 80 percent of imports and 60 percent of exports are handled by Spanish ports.
Aside from the new strikes called on Friday, further industrial action has already been set for June 19, 21 and 23.
(Reporting by Robert Hetz, Writing by Sarah White)
