<p>The Yinshan tomb in Shaoxing, Zhejiang Province, is the first excavated royal mausoleum of the Yue State and is of great significance for archaeological research on both the Yue State and the Eastern Zhou Dynasty. However, extensive tomb robbing has left no direct physical evidence in the tomb that could clearly indicate its construction date or the identity of its occupant, leading to long-standing debates. In this study, we conducted tree-ring dating of a diagonal brace timber from the sidewall of the tomb’s wooden chamber. We established a floating tree-ring width series spanning 178 yr and a tree-ring oxygen isotope (<i>δ</i>¹⁸O) series spanning 154 yr. The tree-ring <i>δ</i>¹⁸O floating series was then cross-dated against the Yangzhou <i>Cunninghamia lanceolata</i> tree-ring <i>δ</i>¹⁸O master chronology. The result revealed that the outermost ring of the samples formed in 490 BCE, the 7th year of the reign of King Goujian of Yue. This finding was further verified through radiocarbon wiggle-matching of five tree rings with known time gaps from the same sample. Given the cutting and decay observed on the outer edge of the timber, the tomb was likely constructed sometime after 490 BCE. Yunchang, the first king of Yue and father of Goujian, died in 497 BCE; therefore, the possibility that Yunchang was the tomb owner can be largely ruled out. However, whether the occupant was Goujian or a later king of Yue requires further investigation. This study demonstrates the effectiveness and potential of tree-ring <i>δ</i>¹⁸O cross-dating in archaeological dating in the humid southeastern region of China, indicating that it can serve as a reliable and routine technique for high-precision dating of archaeological wood in the future.</p>