How do extremist sympathizers respond to counter-radicalization efforts? Over the past decade, programs to counter violent extremism have mushroomed around the world, but little is known of their effectiveness. This study uses social media data to examine how counter-radicalization efforts shape engagement with extremist groups in the online world. Matching geolocated Twitter data on Islamic State sympathizers with granular information on counter-extremism activities in the United States, the study finds that, rather than deradicalizing, these efforts led Islamic State sympathizers to act strategically to avoid detection. After counter-extremism activities, the group’s supporters on Twitter who were in the vicinity of these events began self-censoring expressions of support for the Islamic State, altered profile images and screen names, and encouraged followers to migrate to Telegram, an encrypted network not viewable by the public. These findings reveal previously unknown patterns in the effects of counter-extremism programs in the digital era.