Michael Riffaterre’s semiotic theory, with its emphasis on ungrammaticality and multi-layered reading, has provided an influential model for analyzing the distinctive structure of poetic language. This study investigates the application of Riffaterre’s semiotic framework to Qur’anic interpretation, with a specific focus on Angelika Neuwirth’s intertextual readings of the Qur’an. Neuwirth views the Qur’an as a poetic and dialogical text that engages with earlier religious and cultural traditions. She employs Riffaterre’s model to reveal the text's semantic depth and internal coherence through the notions of ungrammaticality and dual signification. Using Surah al-Ikhlāṣ as a case study, this paper critically evaluates Neuwirth’s application of Riffaterre’s theory by examining her treatment of the supposed “ungrammaticality” regarding the use of the word aḥad. The paper argues that the notion of ungrammaticality in the Qur’an can be reinterpreted not as a violation of linguistic norms, but rather as a semiotic cue that signals deeper intertextual and theological meanings. Accordingly, evaluating alleged irregularities requires a contextual analysis of lexical patterns across the entire Qur’an, where usage, frequency, and semantic range reveal a consistent theological logic. By integrating insights from classical Arabic grammar, lexicography, and tafsīr with modern semiotic theory, this study reassesses the scope and limits of applying Riffaterre’s model to sacred text analysis. It concludes that while semiotic and intertextual approaches can illuminate the Qur’an’s structural and semantic complexity, they must operate within a balanced hermeneutical framework that respects the text's revelatory nature and linguistic precision. Furthermore, this study demonstrates that the intentional utilization of ungrammaticality in the Qur’an effectively serves specific theological functions.