Governments have long worried about terrorists using the Internet to launch cyberattacks, spread propaganda, recruit and radicalise individuals and raise funds. However, the Islamic State’s exploitation of social media has caused a crisis and generated questions about international law’s role in addressing terrorism in cyberspace. This article analyzes international law in connection with potential terrorist cyberattacks and terrorist use of cyber technologies for other purposes. International law is not well positioned to support responses to terrorist cyberattacks, but the lack of such attacks to date undermines incentives for states to develop international law against this threat. In terms of terrorists using the Internet and social media for propaganda, radicalisation, recruiting and fundraising, the crisis caused by the Islamic State’s online activities has not created consensus strong enough to support a prominent role for international law in countering cyber-facilitated terrorism.