Three varieties of Kusunda, a moribund language isolate of Nepal, have been recorded in existing literature; in Hodgson (1857), in Reinhard & Toba (1970), and in several recent publications analyzing material elicited from the language’s last two fluent speakers, Gyani Maiya Sen and Kamala Khatri. Each of these varieties exhibits a set of unique phonological and morphological innovations from their latest common ancestor, Proto-Kusunda (PK). This paper seeks to reconstruct the prefixing possessive marking system of PK, using morphological evidence from the 3 attested varieties. Proto-Kusunda is found to have exhibited obligatory possessive marking on a set of inalienably possessed nouns. Possessed nouns were marked with 2 sets of preposed affixes: *t- *n- *g-, which indexed the person of the noun’s possessor, and *-i- *-a- *-u- *-ja-, a set of derivational prefixes which categorized possessed nominals into a number of semantic fields. The formal and functional characteristics of this system are strongly reminiscent of an analogous system of head-marking possession found in the Great Andamanese language family of India, prompting questions of possible areal influence or genetic inheritance in the remote past.