quality:corruption, structural:reformatting
Return to politics (2015–2018)

Mahathir repeatedly called for prime minister Najib Razak to resign over the 1Malaysia Development Berhad (1MDB) corruption scandal. On 30 August 2015, he and Siti Hasmah attended the Bersih 4 rally, a mass protest organised in response to the scandal. In 2016, Mahathir chaired the Malaysian Citizens' Declaration, which brought together several political figures and non-governmental organisations in calling for Najib's resignation.

Mahathir left UMNO in 2016, and formed Parti Pribumi Bersatu Malaysia (BERSATU). The new party was officially registered on 9 September 2016, and Mahathir became its chairman. By 2017, he had officially joined the opposition coalition Pakatan Harapan. Negotiations then took place between different factions of the coalition for Mahathir to become the chairman and prime ministerial candidate. He assumed the position of chairman on 14 July 2017, despite reservations from supporters of Anwar Ibrahim, who could not contest in polls himself while imprisoned.

In 2017 Mahathir referred to Najib as "a prime minister who came from 'Bugis pirates'" and remarked, "go back to Sulawesi ". Bugis people in Malaysia and Indonesia criticised his language and protested against him.

In early 2018, Mahathir was announced as Pakatan Harapan's prime ministerial candidate for the upcoming general election. Wan Azizah, wife of his former political enemy Anwar, ran as his deputy. Mahathir's election promise was to seek a pardon for Anwar, in order to allow him to take over as prime minister after an interim period.