Harm Kolthek Jr. (1872-1946) was a Dutch printer, journalist, trade unionist and libertarian socialist politician.

Kolthek led the syndicalist federation of trade unions, the National Labor Secretariat (NAS), from 1907 to 1913.

At the age of twenty, he became involved in socialism after his later wife's brother was arrested and abused by hussars at a worker's revolt in Sappemeer; the brother-in-law succumbed to his injuries.

He joined forces with Willy Kruyt of the League of Christian Socialists, but the cooperation did not last.

He returned to (local) politics in 1931, getting elected to the Groningen municipal council on behalf of the Rights and Freedom party (Dutch: Recht en Vrijheid), which he had founded.