Quilliam was a British think tank co-founded in 2008 by Maajid Nawaz that focused on counter-extremism, specifically against Islamism, which it argued represents a desire to impose a given interpretation of Islam on society. Founded as The Quilliam Foundation and based in London, it claimed to lobby government and public institutions for more nuanced policies regarding Islam and on the need for greater democracy in the Muslim world whilst empowering "moderate Muslim" voices. The organisation opposed any Islamist ideology and championed freedom of expression. The critique of Islamist ideology by its founders―Nawaz, Rashad Zaman Ali and Ed Husain―was based, in part, on their personal experiences. Quilliam went into liquidation in 2021.

Quilliam was established in 2007 by Ed Husain, Maajid Nawaz and Rashad Zaman Ali, three former members of the Islamist group Hizb ut-Tahrir. Husain left in 2011 to join the Council on Foreign Relations in New York. Douglas Murray, who founded the Centre for Social Cohesion (which later morphed into the Henry Jackson Society), claimed: "Around the time Ed Husain came to public notice, I recruited him to work with me (through Civitas, the organisation that originally hosted the CSC). He liked my views and I had great hopes for him to become a source for real reform. This gave him the time and financial freedom to set up [Quilliam]."

The organisation was named after Abdullah Quilliam, a 19th-century British convert to Islam who founded Britain's first mosque. The organisation was originally called The Quilliam Foundation, but later rebranded as simply Quilliam.

Quilliam defined Islamism in the following terms:

It is the belief that Islam is a political ideology, as well as a faith. It is a modernist claim that political sovereignty belongs to God, that the Shari'ah should be used as state law, that Muslims form a political rather than a religious bloc around the world and that it is a religious duty for all Muslims to create a political entity that is governed as such. Islamism is a spectrum, with Islamists disagreeing over how they should bring their 'Islamic' state into existence. Some Islamists seek to engage with existing political systems, others reject the existing systems as illegitimate but do so non-violently, and others seek to create an 'Islamic state' through violence. Most Islamists are socially modern but others advocate a more retrograde lifestyle. Islamists often have contempt for Muslim scholars and sages and their traditional institutions; as well as a disdain for non-Islamist Muslims and the West.

Quilliam argued that Islam is a faith, not an ideology, and that "Islam is not Islamism". It also argues that "[Islamists] are extreme because of their rigidity in understanding politics".

The organization's goals were mainly communicated in three ways: through the publication of reports, through involvement with the media, i.e. by taking part in interviews and discussions across Europe and the Middle East, and through its "Outreach and Training" unit, which delivers a "radicalisation awareness programme".

On 30 December 2008, just days after the outbreak of the Gaza War, Husain condemned the "ruthless air strikes and economic blockade" of Gaza city by Israel. He predicted that the result would be "rightful support for the beleaguered Palestinian peoples – and a boost to the popularity of Hamas by default".