Konstantin "Kostja" Zetkin (14 April 1885 – September 1980) was a German physician, social economist and political activist.

Kostja's father, Ossip Zetkin (1850–1889), was a Russian revolutionary and socialist who had suffered persecution on account of his involvement in the Narodniks movement and fled to Leipzig where, as a young man, he had supported himself as a carpenter and become active in student politics.

In the context of the recently enacted Anti-Socialist Laws Ossip Zetkin was arrested at a political meeting in 1880, identified as a "burdensome foreigner" ("lästiger Ausländer") and deprived of his Leipzig residence permit.

[1] According to one source they could not marry because Ossip was unable to obtain the necessary papers from Russia: another version indicates that Clara was reluctant to enter into a marriage which would have caused her to lose her German citizenship.

Instead of returning to Saxony, where she had been born and grown up, she took her family to live in the west of the country, in Stuttgart, where she remained till the mid-1920s, and where Kostja and his elder brother grew up.

They both attended the well-regarded Karls-Gymnasium (secondary school) in Stuttgart,[1] while their mother energetically pursued a political and journalistic career as an activist member of the Social Democratic Party (SPD) and as editor of Gleichheit, a bimonthly women's newspaper committed to gender equality.

[1] During the years after 1923 his mother's health deteriorated progressively and he devoted a substantial part of his energy to looking after her and to supporting her political work.

His mother remained a prolific writer, and her letters disclose that, during the 1920s and early 1930s, Kostja Zetkin was living for a time with Nadja von Massov.

[7] After the decline of the Weimar Republic, the Nazis took power in January 1933 and lost little time in transforming Germany into a one- party dictatorship.

As a high-profile communist, friend of Rosa Luxemburg and feminist activist over many decades, Clara Zetkin enjoyed a quasi-iconic status with Moscow power brokers, which now proved to be something of a mixed blessing.

Kostja Zetkin found himself in disagreement with government representatives, apparently because of differing opinions concerning the selective publication of some of his late mother's large collection of articles, essays, letters and other politically relevant papers.

The authorities responded by identifying as enemy aliens thousands of German refugees who had been forced to seek refuge in France for reasons of politics and / or race.

"[8] Even if the story has been embroidered in the telling, it is beyond dispute that Kostja and Gertrude succeeded, with difficulty, in escaping via Spain (and probably Portugal) to the United States, where they arrived in or before 1945.

As McCarthyism became mainstream in the political establishment, the fact that Kostja's elder brother, having survived in Moscow the Stalinist purges of the later 1930s, was now back in Germany working as a hospital director and senior professor of medicine at the principal university in Soviet administered East Berlin, will have done nothing to enhance Kostja's own career prospects in the US.

The Zetkins now relocated one last time, settling in a cottage owned by one of Gertrude's sisters on the Canadian west coast at Halfmoon Bay.

[8] By this time Kostja Zetkin's health was failing, and in 1963 he had to undergo a major operation, which involved the removal of most of his stomach, on account of a perforated ulcer.

In 2008, to the bewilderment of some scholars, Rosa Luxemburg's life formed the basis for a stage-musical at the Grips-Theater (youth theatre) in Berlin.