Helene or Helena Demuth (30 December 1820 – 4 November 1890) was a German housekeeper who worked for Jenny von Westphalen and Karl Marx, and later served as the household manager and political confidante of Friedrich Engels.

[1] The pair worked in tandem to organize and arrange for the publication of Marx's literary estate, discovering in the process the manuscript from which Engels was able to reconstruct the second volume of Das Kapital.

The obituary penned by Engels in The People's Press writes the following: "The leaders of the Socialist movement bore testimony to "her strong common-sense, her absolute rectitude of character, her ceaseless thoughtfulness for others, her reliability, and the essential truthfulness of her nature".

Engels at her funeral declared that Marx took counsel of Helena Demuth, not only in difficult and intricate party matters, but even in respect of his economical writings.

The child's paternity, however, remains a subject of discussion, with the academic Terrell Carver stating that, although it has been claimed since 1962 that Marx was the father, "this is not well founded on the documentary materials available", adding that "the gossip" is not supported by "direct evidence that bears unambiguously on this matter".