Stern is just one of a number of Hatnua MKs who left the party following Livni's merger with the Labor party.
Together with Yesh Atid Chairman Yair Lapid, Stern held a press conference to discuss the move at Beit Sokolov in Tel Aviv.
"Stern was basically the 20th Knesset member of Yesh Atid," Lapid said. Yesh Atid won 19 seats in the 19th Knesset which dissolved in December.
"He worked with us side by side. We are both the children of Holocaust survivors and we both worked for survivors. Under Stern's yarmulke is a general's beret."
"We stand together because we both believe it is possible and necessary to prevent the nation from splitting into tribes. We both joined politics to bring control back to the people," Lapid continued. "The State belongs to the middle class and the disadvantaged."
Taking the microphone next, Stern discussed his journey into politics.
He grew up in a religious Zionist household and was obviously asked to join the Jewish Home party, but "unfortunately, most of the barriers to creating an inclusive Judaism came from my friends in Jewish Home and I didn't understand why," Stern explained.
"I've always found shelter inside Yesh Atid," Stern added, noting that without Lapid's party, "the conversion bill and the government's decision on the subject would not have been reached.
Stern was the original author of the controversial conversion bill which made headlines last fall.
Following his resignation from Hatnua last month, Stern sent a letter to Tzipi Livni outlining his reasons for leaving, telling her he was "re-thinking" and "examining the correct way to continue to serve the public and the state," either from within or without the political field.
"After I listened to you most attentively and in the light of your recent conduct, I understand even more that my natural place is not in Hatnua, the way it is developing, and it is apparent that I would face much difficulty in continuing my public service within this framework," he noted.
Livni responded by saying that she "respects Stern's decision to leave," adding that "Stern is a person of values" who had "worked to progress an open and accessible Judaism, and I am confident that he will be able to give to the State of Israel in whatever place he will be."
It is as yet unclear what spot Stern will be allocated on Yesh Atid's list for the next Knesset; the centrist party is expected to drop from its current 19 seats to anything from 10-12 mandates, according to current polls.