In Defense of Women is H. L. Mencken's 1918 book on women and the relationship between the sexes. Some laud the book as progressive while others brand it as reactionary. While Mencken did not champion women's rights, he described women as wiser in many novel and observable ways, while demeaning average men.

In general, biographers describe Defense as "ironic": it was not so much a defense of women as a critique of the relationship between the sexes. Topics covered by the book included "Woman's Equipment," "Compulsory Marriage," "The Emancipated Housewife," and "Women as Martyrs." Women were gaining rights, according to Mencken—the ability to partake in adultery without lasting public disgrace, the ability to divorce men, and even some escape from the notion of virginity as sacred, which remained as "one of the hollow conventions of Christianity." Women nonetheless remained restrained by social conventions in many capacities.

Mencken's love of women was driven in part by the sympathy he had for female literary characters (especially those brought to life by his friend Theodore Dreiser), as well as his almost fanatical love of his mother. Mencken supported women's rights, even if he had no affection for the suffragist.

Although he originally intended to be ironic when he proclaimed that women were the superior gender, many of the qualities he assigned to them were qualities he deeply admired – realism and skepticism among them, but also manipulative skill and a detached view of humankind.

Mencken praised women, though he believed they should remain in the background of industry and politics. In personal letters especially, Mencken would write that women should appreciate men and do their best to support them.

Mencken often espoused views of politics, religion, and metaphysics that stressed their grotesqueness and absurdity; in this context, escape from the supposed fraud of such somber subjects was welcome to him.