Pada prasasti Gunung Tua (Lokanātha) ditemukan sejumlah kata yang mengindikasikan bahasa Proto-Austronesia. Jejak bahasa Proto-Austronesia tersebut berupa kata turunan dan/atau kalimat-kalimat, yang dapat dianalisis pada tataran fonologi, morfologi, dan sintaksis. Permasalahan penelitian ini adalah mengetahui tataran fonologi dan morfologi bahasa Proto-Austronesia pada prasasti Gunung Tua (Lokanātha), dan tataran sintaksis dari struktur kalimat bahasa Melayu kuno pada prasasti tersebut. Tujuan penelitian ini untuk memahami signifikasi bahasa Proto-Austronesia pada Prasasti Gunung Tua (Lokanātha). Metode penelitian ini bersifat kualitatif yang dijabarkan secara deskriptif, serta analisis data yang dilakukan dengan metode padan dan agih. Hasil penelitian menunjukkan bahwa pada tataran fonologi, terdapat dua kata, yaitu juru atau ‘orang pandai’, dan pāṇḍai atau ‘pandai (terampil)’. Pada tataran morfologi, ditemukan dua kata, yaitu tatkala atau ‘ketika’, dan barbwat atau ‘membuat’. Ternyata, kata juru dan tatkala merupakan kata serapan dari bahasa Sanskerta, sedangkan kata pāṇḍai dan barbwat merupakan kata turunan dari bahasa Proto-Austronesia. Berdasarkan tataran sintaksis, dapat disimpulkan bahwa struktur kalimat pada prasasti Gunung Tua (Lokanātha) berbentuk kalimat aktif transitif atau kalimat yang memerlukan objek. The Gunung Tua (Lokanātha) inscription contained words indicating the Proto-Austronesian language. Traces of the Proto-Austronesian language were evident from derivative words and/or sentences written in the inscription, and these could be analyzed at levels of phonology, morphology, or syntax. This research was carried out to recognize the phonological and morphological level of the Proto-Austronesian language on the Gunung Tua (Lokanātha) inscription, and the syntactic level of sentence structure of the ancient Malay language. The purpose of this study was to understand the significance of the Proto-Austronesian language in the Gunung Tua (Lokanātha) inscription. This research method is qualitative, and data were portrayed descriptively. Data analysis was carried out using methods of identity and distributional. Research results showed that at the phonological level, two words were recognized, which were ‘juru’ or smart person, and ‘pāṇḍai’ or smart (skilled). At the morphological level, two words were identified, which were ‘tatkala’ or when, and ‘barbwat’ or make. Apparently, the words ‘juru’ and ‘tatkala’ were loanwords from Sanskrit, while the words ‘pāṇḍai’ and ‘barbwat’ were derivative words of the Proto-Austronesian language. At a syntactic level, it could be concluded that the sentence structure of the Gunung Tua (Lokanātha) inscription was transitive active.