The announcement was made at a press conference held at his office at the Perdana Leadership Foundation in Putrajaya this afternoon.
"My decision to leave UMNO is because it is no longer UMNO, it is a party dedicated to support Najib (Datuk Seri Najib Razak), protecting Najib and upholding what he does.
"I cannot be a member of such a party," the former premier said.
Dr Mahathir said his wife, Tun Dr Siti Hasmah has also quit the party effective immediately.
He stressed that in any case, he would not rejoin UMNO nor would he join any Opposition party or start a new political party.
"No. In my heart, I am still with UMNO," he said.
When asked if there would be anyone else following his footsteps soon, he declined to elaborate.
When asked if he had submitted an official letter to the party's secretary-general on his decision, Dr Mahathir said he has yet to do so.
Dr Mahathir has been among the harshest critics of Najib, who is Prime Minister and UMNO president, and has spoken out against the latter numerous times, especially via his blog site.
He is currently under police investigation for alleged slander, and had underwent police questioning last Wednesday.
READ: Police finalising investigation on Dr Mahathir
After the press conference, a two-page media statement was distributed to media members stating issues which Dr Mahathir had raised on Najib before.
The authenticity of the statement, however, is unverified.
'UMNO is no longer UMNO'
'Malaysia is in a very bad state'
'I don't practice nepotism'
This is the second time that Dr Mahathir has quit UMNO, with the first being on May 19, 2008 when he claimed to have lost confidence in the leadership of then prime minister and UMNO president Datuk Seri (now Tun) Abdullah Ahmad Badawi.
Dr Mahathir rejoined the party on April 4, 2009 when Najib assumed the national and party leadership.
On Sept 26, 1969, Dr Mahathir was expelled from UMNO for disciplinary reasons, during Tunku Abdul Rahman Putra Al-Haj's administration and returned to the UMNO fold on March 7, 1972 when Tun Abdul Razak Hussein took over.
