Speculative grammar is a fourteenth-century movement in medieval philosophy, which assumed that the analysis of the grammar of ordinary language was the key to metaphysics.

The speculative grammarians or modistae transformed metaphysics into ordinary-language philosophy. They assumed that grammatical forms, the "modi significandi" of common verbs, nouns, and adjectives, indicate deep ontological structure. The modistae affirmed against nominalism an ontic foundation of language, disengaged through grammatical analysis. Roger Bacon inspired the movement with his observation that all languages are built upon a common grammar, a shared foundation of ontically anchored linguistic structures: Grammar is substantially the same in all languages, even though it may undergo in them accidental variations.

The most famous treatise on speculate grammar, De modis significandi sive grammatica speculativa was written by Thomas of Erfurt in the fourteenth century. Until the early twentieth-century it was assumed to be by John Duns Scotus. Widely produced and commented upon in the Middle Ages, it is the most complete speculative grammar extant. The mistaken authorship arose out of the natural affinity of Erfurt's speculative grammar with Scotus's metaphysics. For Erfurt, as for Scotus, being (ens) means essence (essentia); hence a complete account of the modes of meaning, the categories of categories, is assumed to exhaust being.

The movement was influenced by the philosophy of Duns Scotus, which held that every level of experience is permeated by understandability, by essentia. The sheer undefinable and unnameable 'thisness' of experience, haecceitas, is not a material residue outside the intelligible sphere of essentia, it is not Aristotle's potentia, nor Aquinas's existentia -- but rather, the most determinate and concrete mode of essentia. Although it could not be directly defined, this deep intelligibility leaves traces in ordinary language. There are parallels between speculative grammar and phenomenology, a fact that was picked up early on by Martin Heidegger, who wrote his first book, Die Kategorien-und Bedeutungslehre des Duns Scotus, on Thomas of Erfurt's treatise.