Empty space in regional shopping malls reached a six-year high in the first quarter, adding further stress to regions being hit by a retail earthquake that is shaking up the job market across the U.S.

The vacancy rate in big U.S. malls increased to 8.4% in the first quarter of 2018, up from 8.3% in the fourth quarter and the highest since the fourth quarter of 2012, according to real-estate data firm Reis Inc., which studies 77 metropolitan areas. Meanwhile, neighborhood and community shopping centers in 41 of the 77 areas...