Graeco-Aryan (or Graeco-Armeno-Aryan) is a hypothetical clade within the Indo-European family, ancestral to the Greek language, the Armenian language, and the Indo-Iranian languages. Graeco-Aryan unity would have become divided into Proto-Greek and Proto-Indo-Iranian by the mid 3rd millennium BC. Conceivably, Proto-Armenian would have been located between Proto-Greek and Proto-Indo-Iranian, consistent with the fact that Armenian shares certain features only with Indo-Iranian (the satem change) but others only with Greek (s > h).
Graeco-Aryan has comparatively wide support among Indo-Europeanists for the Indo-European Homeland to be located in the Armenian Highland. Early and strong evidence was given by Euler's 1979 examination on shared features in Greek and Sanskrit nominal flection.
Used in tandem with the Graeco-Armenian hypothesis, the Armenian language would also be included under the label Aryano-Greco-Armenic, splitting into proto-Greek/Phrygian and "Armeno-Aryan" (ancestor of Armenian and Indo-Iranian).
In the context of the Kurgan hypothesis, Greco-Aryan is also known as "Late PIE" or "Late Indo-European" (LIE), suggesting that Greco-Aryan forms a dialect group which corresponds to the latest stage of linguistic unity in the Indo-European homeland in the early part of the 3rd millennium BC. By 2500 BC, Proto-Greek and Proto-Indo-Iranian had separated, moving westward and eastward from the Pontic Steppe, respectively.
If Graeco-Aryan is a valid group, Grassmann's law may have a common origin in Greek and Sanskrit. Note, however, that Grassmann's law in Greek postdates certain sound changes that happened only in Greek and not Sanskrit, which suggests that it cannot strictly be an inheritance from a common Graeco-Aryan stage. Rather, it is more likely an areal feature that spread across a then-contiguous Graeco-Aryan–speaking area after early Proto-Greek and Proto-Indo-Iranian had developed into separate dialects but before they ceased being in geographic contact.
Graeco-Aryan is invoked in particular in studies of comparative mythology, e.g. by West (1999) and Watkins (2001).