"I am now going full-time into politics as a member and a revolutionary cadre of the Economic Freedom Fighters movement to make it a force to be reckoned with and to contest the next election," Kunene wrote in a letter which he tweeted.
The sushi king also said "the life of sushi and my army of women I put behind me".
Kunene wrote an open letter to President Jacob Zuma last month, accusing him of being heartless and controlled by the Guptas - the wealthy family who managed to land their chartered plane full of guests at the Air Force Base Waterkloof.
"In public you smile and laugh, but in truth you behave like a monster, a tyrant who will target perceived enemies ruthlessly, and because of that fear, few dare to speak openly," the letter read.
The ANC hit back at Kunene without mentioning him by name.
ANC spokesperson Jackson Mthembu said the governing party views "such statements as disrespectful, distasteful and insulting to the person of the president and the ANC".
City Press reported last month that Kunene was tipped to become one of the faces of the Economic Freedom Fighters (EFF).
Kunene, the forum's spokesperson Floyd Shivambu, and Malema have been close allies.
In today's letter, the businessman said the ANC could attack him but "the fact is that personally attacking the people who criticise you can't make the criticisms they may have about you disappear".
On why he was joining the EFF, Kunene wrote: "The answer is that the majority of people I have interacted with are telling me we need an alternative voice in this country that will be revolutionary."
"The formation of the Economic Freedom Fighters movement is therefore not the decision of Julius Malema, Floyd Shivambu or Kenny Kunene."
"It is the better, alternative voice of South Africans who are fed up with living without hope that things will ever change for the better," he said.