The Scarlet Letter is—to a certain extent—a novel that works in multiple genres. First and foremost, it makes sense to call the novel a work of historical fiction, since it was published in 1850 but takes place in Puritan New England in the 1600s. An obvious aim of the novel is to explore the mores and taboos of Puritan society, looking at the hypocrisy and severity of New England society at the time and inviting 19th-century (or contemporary) readers to experience what it might have been like to live through this unforgiving period.
In keeping with its examination of this severe historical setting, The Scarlet Letter also presents readers with an unsettling atmosphere that ultimately aligns with the conventions of Gothic literature. An interest in darkness, nature, and other troubling elements marks the novel as a work of Gothic fiction, as do a number of similes or metaphors throughout the novel that make comparisons to supernatural phenomena—a staple of the Gothic genre.
Lastly, the novel is also a love story, as made quite clear by the full title: The Scarlet Letter: A Romance. Although it's important to distinguish between romance novels and works that adhere to the Romantic literary genre, it's worth noting that The Scarlet Letter was published during the period most frequently associated with American Romanticism. Furthermore, its interest in its characters strong—even violent—emotions (which are often repressed, as is the case for Arthur Dimmesdale) aligns the novel with Romantic sensibilities, since Romanticism placed a strong emphasis on human emotion.
