Barak's daughters, Priel's children and a small number of close friends were invited to the private wedding which will be held at 1 pm at the couple's luxury apartment in Tel Aviv.
The ceremony will be conducted by Rabbi Shmuel Rabinovitch, rabbi of the Western Wall.
Nili Priel sounded excited on Thursday but refused to give away any details on the wedding. The happy groom also refused to talk.
A highly decorated war hero, Barak, 65 was elected as Israel's 10th prime minister in 1999. His wife at the time, Nava, was popular with the public and was widely credited with softening his gruff, aloof persona.
Barak and Priel, 63, met when they were in their youth, but went separate ways. Barak, who was a promising officer at the General Staff Reconnaissance Unit, married Nava and the two had three daughters together. Priel also got married and became a successful public relations executive.
In 2003, the two were reacquainted and decided to leave their spouses. Barak shocked the country when he decided to leave home and dissolve what was considered one of the most stable marriages in Israeli politics.
Nava Barak, who was also surprised by her husband's decision, turned into an eligible bachelorette. She is currently in a relationship with businessman Shalom Zinger.
Barak ended his political exile earlier this year, recapturing the leadership of the Labor Party and taking over the Defense Ministry.
Analysts predicted the wedding was another step in Barak's attempted comeback to Israel's top post. Israel has never had a divorced bachelor as prime minister, though Benjamin Netanyahu was married to his third and current wife when he was elected in 1996. Ariel Sharon was already widowed when he came to power in 2001.