The mausoleum of Imāmzāda ʿAbdallāh (Markazī province, Iran) has been attributed to the period between the 10th to 12th century AD. Its interior surfaces have been understood to be decorated in gypsum-based stuccos, but this research reveals the existence of hitherto unexpected compositions. Despite its significance for the history of Iranian architecture, this monument still lacks thorough scrutiny of its architecture and decoration. This study presents new insights into the monument’s history and unique artistic production of its architectural decoration. It includes a full documentation of inscriptions, a detailed plan, and sections. Furthermore, it proposes a technological and stylistic comparison of the mausoleum’s revetments with Seljuk stuccos. Finally, the results of decorative materials’ analytical study reveal the nature of its decorative materials.