The Proto-Indo-European causative/iterative suffix *-ei̯e- was inherited by Old Iranian and persists in almost all Middle and Modern Iranian languages as -aya- and -ēn- (-Vn-) respectively. Comparably, in the Indic branch -aya- functions as a causative suffix in Sanskrit beside another suffix -āpaya which became the productive causative suffix -āvē- in Middle Indic and still used in Modern Indic today. Evidence shows eight Eastern Iranian languages- †Khotanese, †Khwarazmian, Parachi, Wakhi, Munji, Pashto, Ormuri, and Yidgha- using the morphological causative suffix in addition to the expected Iranian one -aya- or -Vn-. This alternative causative suffix is reconstructible as *-au̯ai̯a- and its attested reflexes have the forms -VwV-, -Vv-, and -wV-. Moreover, in two dialects of the Northwestern Iranian language Gilaki, Dakhili and Langaroudi, the causative suffix is not -Vn- but is rather -be(ː)- in the present tense. In this study I examine the synchronic function of the Gilaki causative suffix -be(ː)- as well as its diachronic origins. I show that Gilaki -be(ː)- primarily functions as a causative suffix and that it is a form which cannot be explained as an innovation within Gilaki itself through phonological or analogical change. As a matter of fact, I demonstrate that this suffix is better explained as deriving from PIr.*-au̯ai̯a- and is connected to the aforementioned Eastern Iranian suffixes. I also argue the reason for realization of /p/ and */u̯/ in -āpaya and *-au̯ai̯a- is phonological and probably goes back to some stages of PIIr.