Whilst Article 51 of the UN Charter indicates that an ‘armed attack’ may trigger a State’s inherent right of individual or collective self-defence, the purport of armed attack remains a matter of interpretation and qualification. Moreover, actions carried out in (or through) cyberspace have caused the impetus for another debate: whether and when cyber-attacks can be qualified as an armed attack. To improve the notion of self-defence and contribute to the jus ad bellum (international law on the use of transnational force), more clarification as to what constitutes an armed attack in cyberspace is necessary. The main aim of the paper is to propose a tangible guideline that outlines when cyber-attacks – perpetrated solely in or through cyberspace and not in conjunction with conventional military attacks – can be qualified as an armed attack.