Post-premiership
Since his resignation as Prime Minister and loss of his party chairmanship, Sigmundur Davíð has repeatedly asserted that he was the victim of a global conspiracy to bring him down. He has implied that George Soros, "banking elites", the Swedish public broadcaster, the International Consortium of Investigative Journalists and the Icelandic public broadcaster conspired against him. In February 2017, Sigmundur Davíð said that there had been a "hostile takeover" of the Progressive Party and that the current leadership of the party no longer reflects the will of the majority of the party members. In March 2017, Sigmundur Davíð further alleged that the SVTs Uppdrag granskning interview was "falsified", and that the interviewers had practised how to confuse him as much as they could.
In September 2017, after the announcement of a snap election, Sigmundur Davíð stated in an open letter on his website that he was seeking to form a new political party before the 2017 Icelandic parliamentary election. His new party, the Centre Party Miðflokkurinn, has been described as "populist". Miðflokkurinn finished fifth in the 2017 parliamentary election with seven candidates, including Sigmundur Davíð, being elected to the Althing. It received 10.9% of the vote, slightly ahead of the Progressive Party. Since 2017, Sigmundur Davíð has been a member of the Icelandic Division of the International Parliamentary Assembly.
In December 2018, a leaked recording captured four Centre Party MPs, including Sigmundur Davíð, mocking a disabled woman and other women using denigrating and sexually charged language. The scandal became known as the Klaustur Affair.
Sigmundur has spoken strongly against the European Union's third energy packet and argued that its main purpose would be EU domination of Icelandic energy sources. He also argued that the third energy packet would be in contradiction of the Icelandic constitution. In the spring of 2019 the Centre party held filibuster speeches for over 130 hours about the subject in the Althing. In July 2019, he criticised the Paris Climate Accords as pointless, despite having signed it himself during his premiership, and argued that actions taken to curb climate change will not have an impact.
In 2020, he characterised the Black Lives Matter movement as racist and compared that year's Black Lives Matter protests in the United States to the Cultural Revolution in China.
His Centre party faced a big defeat in the 2021 parliamentary election, only receiving 5.4% of the popular vote and electing three MPs. One of those MPs left the party to join the Independence Party in October 2021, leaving only two Centre Party MPs, including Sigmundur, in parliament. In polls leading up to the snap election in 2024, the party ranged from 18.7% to 11.1%, ultimately receiving 12.1% of the vote and electing eight MPs, up from two MPs. As of December 2025, the Centre Party is polling at 19.5% support nationwide.