The Aristolochiaceae (English: /əˌrɪstəˈloʊkiəsii/) are a family, the birthwort family, of flowering plants with seven genera and about 400 known species belonging to the order Piperales. The type genus is Aristolochia L.

They are mostly perennial, herbaceous plants, shrubs, or lianas. The membranous, cordate simple leaves are spread out, growing alternately along the stem on leaf stalks. The margins are commonly entire. No stipules are present. The bizarre flowers are large to medium-sized, growing in the leaf axils. They are bilaterally or radially symmetrical.

Aristolochiaceae are magnoliids, a basal group of angiosperms which are not part of the large categories of monocots or eudicots. As of APG IV (2016), the former families Hydnoraceae and Lactoridaceae are included, because exclusion would make Aristolochiaceae in the traditional sense paraphyletic.

Some newer classification schemes, such as the update of the Angiosperm Phylogeny Group, place the family Aristolochiaceae in the order Piperales, but it is still quite common, though superseded, for the Aristolochiaceae to be assigned, sometimes with some other families, their own order (Aristolochiales).

Eight genera are accepted – Aristolochia, Asarum, Euglypha, Hydnora, Lactoris, Prosopanche, Saruma, and Thottea.

Four assemblages can be distinguished in the genus-level cladogram of Aristolochiaceae:

Many members of Aristolochia and some of Asarum contain the toxin aristolochic acid, which discourages herbivores and is known to be carcinogenic in rats. Aristolochia species are carcinogenic to humans.

The complete plastid genome sequence of one species of Aristolochiaceae, Hydnora visseri, has been determined. As compared to the chloroplast genome of its closest photosynthetic relatives, the plastome of Hydnora visseri shows extreme reduction in both size (ca. 27 kilo base pairs) and gene content (24 genes appear to be functional). This Aristolochiaceae species therefore possesses one of the smallest plastid genomes among flowering plants.