Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs).

The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.

The name "Merostomata" derives from the Greek roots μηρός (meros, "thigh") and στόμα (stoma, "mouth"), in reference to the animals' possession of appendages which are mouthparts at their proximal end, but swimming legs at their distal end.

[2] Some more recent analyses suggest the grouping Merostomata is not monophyletic, with Xiphosura being basal to a clade comprising Eurypterida and Arachnida.

[3] However, a 2022 analysis recovered the monophyly of Merostomata, as opposed to a monophyletic Arachnida, with Xiphosura as the only modern representative in a derived position, indicating the convergence of several characteristics that supposedly united the arachnids.