Postlexical prosodic phenomena in Austronesian languages have received relatively little attention, and consequently their patterns remain unknown. This paper aims to bridge this gap by investigating how syntactic structure interacts with postlexical prosodic phenomena in Saisiyat, an endangered language spoken in Taiwan. Several significant findings are made. First, Saisiyat sentential fundamental frequency (F0) patterns are based largely on its original lexical-level word F0 contour. The accent of a content word in sentences usually falls on its ultimate syllable, while trisyllabic or quadrisyllabic content words may sometimes undergo postlexical accentual modifications such as accent spreading, accent fronting, and accent adding. Function words, in contrast, play a role of interpolation as an intermediate site in bridging the F0 of their preceding and following syllables. Second, a yes-no question exhibits substantial prosodic modification by influencing its word preceding sentence-final interrogative particle aj, as compared with its counterpart in a declarative sentence. Third, agents in agent-focus sentences and patients in patient-focus sentences demonstrate higher values with respect to F0 peak, mean F0, and mean intensity. We provide typological explanations for these findings and explore the theoretical implications of postlexical prosodic patterns of Formosan languages in Taiwan.