The Adbhutanirṇayaḥ is an unpublished Sanskrit text of an anonymous author, treasured in the trove of the rare manuscripts of the Manuscript section of the Archival cell of the Krishna Kanta Handiqui Library of the Gauhati University, Guwahati. The term adbhuta means portentous happenings, and it is generally applied in the sense of something that has not happened before. The happenings, which are contradictory to the general norms or order, are, therefore, known as the adbhutas. Thus, the text Adbhutanirṇayaḥ deals with ascertaining the adbhutas, i.e. omens and portents, and the rites to be performed in order to counteract their unfavourable effects. An extensive literature on the adbhutas is already available in the Vedic, Epic and the Purāṇic texts. Among the Vedic tests, the Adbhutabrāhmaṇa, included in the Ṣaḍviṁśabrāhmaṇa of the Sāmaveda, the Kauśikasūtra, the Atharvavedapariśiṣṭa, exhaustively deal with the adbhutas. Likewise, the Agnipurāṇa, the Matsyapurāṇa, the Bṛhatsaṁhitā, the Yājñavalkasmṛti, and such other texts, contain the materials on the omens and portents, at length. The present study on the Adbhutanirṇayaḥ reflects well-marked influence of the Vedic and Purāṇic texts on the contents, incorporated in the text, though it is not directly cited from the Vedic and Purāṇic sources. The study also reveals the author’s sound scholarship in the Vedic - Purāṇic texts on portents.