The government, still the largest employer in the country, is excluded from the list.
With almost 28,000 employees, it has six times more workers than the Post, which tops the list with 4,540 employees as of 1 January 2019.
The Post has climbed to the top of the list from fourth place in 2010.
The ten businesses which make up the list account for almost 10% of all Luxembourg's employment, according to the Fondation Idea report.
Between 2010 and 2019, the top ten changed dramatically due to company mergers, divestitures or modifications to the structures following household demand, said the Fondation.
Notable changes are as follows: