BERLIN—About half a million refugees in Germany received social security benefits in June, almost twice as many as a year earlier, potentially fueling tension among voters over the recent influx of refugees and heaping pressure on Chancellor Angela Merkel.

The Federal Employment Agency said 469,403 immigrants from asylum seekers’ eight most common countries of origin received benefits in June, a 93% increase from a year earlier. The number of Syrians tripled to 292,326 while the number of Iraqis rose by almost a quarter to...