This study examines the grammaticalization of a motion verb, glossed as MOVIMP_G into an imperative marker in two sign languages: German Sign Language (DGS) and Greek Sign Language (GSL). We suggest that MOVIMP_G has transitioned from its original use as a lexical motion verb with the meaning ‘go’ or ‘send’ to an imperative marker in both languages. Using data from the Hamburg DGS Corpus and newly designed elicitation tasks in both DGS and GSL, we demonstrate that MOVIMP_G is systematically used in imperative contexts, either alone or in combination with deictic and non-deictic verbs. While MOVIMP_G retains its lexical meaning in some contexts, it also increasingly functions as a pragmatic marker of commands. Notably, our elicitation data show that, when used as a marker of commands, MOVIMP_G resists embedding, exhibits locative agreement depending on the referent’s location, and is incompatible with first-person subjects. In GSL, MOVIMP_G is restricted to commands, while in DGS it also appears in permissive contexts. These morphosyntactic constraints and the semantic shift from a verb of motion to a marker of imperatives suggest ongoing grammaticalization. This grammaticalization process, which is rare among spoken languages, reflects a (perhaps modality-specific) grammaticalization path in sign languages. Our results thus contribute to a better understanding of grammaticalization across modalities, while emphasizing the unique linguistic properties of sign languages.