Our study examines interpersonal functions enacted through a stance marker in spoken ELF academic discourse. We specifically focus on investigating the functions of "I don't know" in an academic speech event by embracing an interpersonal pragmatics and sociolinguistics perspective to figure out how it contributes to the act of stancetaking as an intersubjective activity. We have examined 14 interactions of doctoral defense discussions from the ELFA corpus. Our detailed discourse analysis of these doctoral defense discussions has revealed five distinctive interpersonal functions of the stance marker "I don't know" allowing speakers to construct their stance and adopt a face-maintaining strategy in the ongoing spoken discourse: prefacing a suggestion, seeking acceptance, hedging/mitigating, checking agreement, and expressing uncertainty. Considering the highly-context dependent and context-regenerated functions of "I don't know," our study attempts to delve into the relational and interpersonal aspect of communication, and thus contributes to research in this strand by disclosing the interpersonal functions of stancetaking as an intersubjective activity with a particular focus on ELF academic discourse.