From Stockholm +‎ -er.

Stockholmer (plural Stockholmers)

1. Someone from Stockholm in Sweden.
   • 1963 August, W. Shepherd, “Stockholm's modern underground railway”, in Modern Railways, page 99:

     Stockholmers are very proud of their brand new underground railway—they even refrain from putting their feet on the opposite seats—and they have every reason for their pride.

Stockholmer (not comparable)

1. Of, from or pertaining to Stockholm.