The country, which welcomed over 68 million foreign tourists last year - its third consecutive year of record numbers - employs around 100,000 hotel maids, according to union estimates.
Over the past two years more and more maids have been challenging their contracts in courts and coming out in the press with tales of exploitation in the world's third most visited country.
Pepita Garcia Lupianez, who has worked for 40 years in the seaside resort of Torremolinos on the Costa del Sol, is one of the leaders of the fight despite enjoying better conditions than most.
She had a full-time contract and earns 1,300 euros ($1,400) per month, far above the minimum wage of 764.40 euros.
"I am almost ashamed when I meet with colleagues employed by subcontractors who have contracts of four to six hours and work in reality eight or ten hours," said Lupianez, 59, a representative with Spain's biggest union, Comisiones Obreras (CCOO).
"Their employers tells them: 'Until you have finished, you can't leave!'".
Lupianez took part in a protest in the southern city of Malaga Thursday against a reform of Spain's labour code in 2012 which maids say has led to lower salaries.
The reform made firing workers easier and cheaper and weakened collective bargaining agreements.
Outsourcing of cleaning to less expensive firms has since become widespread.
"In numerous hotels directly-hired staff have been replaced" by employees of service firms, said Ernest Canada, the author of a book on hotel maids.
Maids who work for such firms are not governed by the collective labour agreement for housekeeping staff, but the one for the cleaning sector, and are paid up to 40 percent less than their peers, according to the CCOO.
"We say: 'Enough exploitation!', said Carolina Martin, a 46-year-old maid in the southwestern city of Seville who has filed a complaint against her previous employer.
"I earned just 700 euros to clean 400 rooms per month, they gave us more or less two euros per room we cleaned," she said.
She now works 30 hours a week at a four star hotel in Seville, earning 618 euros a month. The schedule leaves her in "constant stress" with no time to go to the bathroom during her shifts, she said.
The maids often win their legal battles. Of the 58 collective agreements which have been contested since May 2015, 46 have been annulled, according to Spain's two largest unions, CCOO and the UGT.
Prime Minister Mariano Rajoy's conservative government defends its reform of the labour code, crediting it with a drop in Spain's jobless rate to below 20 percent from a record high 27 percent in 2013.
Spain's hotel and retail sector accounted for nearly half of all jobs created this year, according to a study by Adecco, the world's biggest temp agency.
But most new jobs are temporary. One in three Spaniards is employed on a fixed-term contract and the International Monetary Fund on Tuesday identified this large proportion of short-term contracts as a weakness.
It urged Spain to increase incentives for employers to award staff permanent contracts in a preliminary annual review of Spain's economy.
Maids on short-term contracts are becoming more and more common at hotels at PortAventura, one of the largest theme parks in Europe, near the northeastern city of Tarragona, said Esther Rodriguez, a maid who works there on a permanent contract.
They are "young girls, sometimes from Morocco, Senegal, Nigeria, who earn 300 euros less than us," the 54-year-old said.
The president of AC Hotels by Marriott, Antonio Catalan, made headlines last month when he publicly criticised the labour law reform and its impact on hotel maids.
"Today I can fire someone by paying them compensation equivalent to 20 days' pay for each year worked and then start to outsource. This is what those who exploit maids do," he told a business forum.
The Spanish hotel federation declined to comment on the issue.
Some maids are distributing fliers to hotel clients to explain their plight.
"It's the best way to put pressure on hotel owners," said Angela, 54, who was fired from a big hotel chain for refusing to be outsourced.
She is lobbying to ensure staff working conditions are taken into account when awarding a hotel star ranking, along with criteria such as bed size.