This article identifies just so as a newly emerging purpose subordinator. Using data from the Corpus of Contemporary American English and the Corpus of Historical American English, it traces its development and steady increase in frequency from its first attestation in the mid nineteenth century to the present day. Just so is shown to represent a case of semantic specialization where the purpose meaning wins out over the conditional meaning, thus filling the niche of an informal purpose subordinator and providing an alternative to its multifunctional and semantically ambiguous competitors so that and so. With increasing grammaticalization the just so purpose subordinator also exhibits signs of intersubjectification, being coopted for syntactically independent, interpersonal uses (e.g. just so we're clear) and culminating in the emergence of a new discourse marker in the form of just so you know in the late twentieth and early twenty-first century. To account for the emergence of purpose just so, a constructional network approach is adopted, which considers the network links to other purpose subordinators, notably so that and so.