This article contributes to the discussions on Internet mobilization and on international social movements’ ability to influence national policy. The event studied is the ‘first Internet strike’ of 18 January 2012 aimed against the SOPA legislation proposed in the USA. Wikipedia’s volunteer editors from all around the world took part in the vote concerning whether Wikipedia should undertake a protest action aimed at influencing American policymakers. Wikipedia editors are shown to share values of the international free culture movement, though experienced editors were also likely to be conflicted about whether taking part in a protest action was not violating the site’s principle of encyclopedic neutrality. Further, Wikipedia’s participation in this protest action allowed non-US citizens to have a visible impact on the US national legislation. As such, Wikipedia can be seen as an international social movement organization, whose 24 hour-long blackout of its popular website was a major factor in the success of the anti-SOPA protests. Wikipedia’s blackout was an expression of an international political opportunity structure in the form of worldwide awareness and protests, which in turn enabled a national political opportunity structure by informing and mobilizing American citizens.