Peres, an 82-year-old former prime minister who has held every major cabinet position, left Labour after his humiliating loss three weeks ago to union leader Amir Peretz in the race for party leader.
Peres said he was supporting Sharon as the person who had the best chance of restarting the peace process with the Palestinians.
"It was not easy, but I made the choice and decided," he said.
Peres' defection was a coup for Sharon's new Kadima Party as the major political factions scramble to snare high-profile supporters before March parliamentary elections.
Many Israelis respect Peres for his decades of service to the country, but some view him warily as a dove and a political opportunist.
Under a reported deal worked out with Sharon, Peres would campaign for the prime minister in the run-up to the elections without officially joining Kadima.
If Sharon wins, Peres would receive a senior cabinet post, either dealing with the peace process or with his pet project to develop Israel's sparsely populated Negev desert and northern Galilee regions.
"I do not believe that it is possible to push forward the peace process in the current political constellation. I believe the most qualified person for this is Ariel Sharon - he will restart the peace process right after the election," Peres said.
Amir Peretz scored an upset win
in the Labour party election
"I decided to join him and work with him."
"My activities in the (Labour) party have come to an end," Peres said, stopping short of formally announcing his resignation from the party.
Peres's Labour colleagues deplored his walkout.
"I feel sorry for Peres who, having done so much for the country, will be remembered for abandoning his home," senior Labour member Ofir Pines-Paz told the YNET news website.
"I feel sorry for Peres who, having done so much for the country, will be remembered for abandoning his home"
Labour member Ofir Pines-Paz
Peres was visibly stunned when trade union chief Amir Peretz, a Moroccan-born Jew, scored an upset victory in the race for a Labour leadership long dominated by Jews of European descent.
Palestinian chief negotiator Saeb Erekat said the election was an internal Israeli affair but added: "I hope that once the elections are over, the Israelis will have chosen a government that is willing to re-engage in a meaningful peace process that will end the occupation and the conflict."