On a chilly winter night, a few dozen people gathered inside a graffiti-clad building in the Carabanchel district of Madrid. Nearly everyone was in the midst of a dispute with their landlord, but these weren’t typical gripes about leaky pipes. They had come to commiserate about the American investment banks and private equity funds that controlled their homes.

Some at this meeting of the Sindicato de Vivienda de Carabanchel (the Carabanchel Housing Union) were fighting eviction orders or skyrocketing rents. Others had lost their homes through mortgage foreclosures. One attendee, Elsa Riquelme, described her yearslong battle to stay in the 600-square-foot apartment where she raised her three sons, which is now owned by Blackstone, the world’s largest private equity firm.

She was far from alone: Over the past decade, Blackstone has become Madrid’s largest private owner of residential real estate, and the second largest in all of Spain. Ms. Riquelme’s apartment is one of 13,000 that Blackstone currently owns in Madrid, and among 19,600 it owns nationwide.

Across Spain, around 185,000 rental properties are now owned by large corporations, half of those by firms based in the United States, according to a review of property registries by the nonprofit Civio. Rental prices have increased 57 percent since 2015 and home prices 47 percent, according to PwC, in large part because the country has failed to build enough homes for its growing population, even as more than 4 million homes sit empty. After the pandemic pushed Spain’s unemployment rate up to 15 percent, evictions nationwide spiked. In Madrid, tenant groups estimate that 20,000 renters in the city currently face the threat of eviction.

Ms. Riquelme, a bookkeeper by trade, emigrated from Chile in 2000 and bought her apartment for 56,000 euros during the housing bubble, making mortgage payments to CaixaCatalunya, a now defunct bank. After she and her husband split, she could no longer keep up, and the bank eventually foreclosed. CaixaCatalunya sought €150,000 ($170,000) in fees and mortgage arrears, then sold her apartment at auction for just €40,000 ($45,000) to a subsidiary of Blackstone.