He will have the responsibility to oversee and manage all corporate communications and brand related activities for the group in India and abroad, Tata Sons said in a statement.
Rajan's is the first major appointment after Cyrus Mistry took over as the Chairman of the group after Ratan Tata retired on December 28, 2012.
"Additionally, he (Rajan) will serve as the Chief Ethics Officer of the group and also oversee all Tata group Corporate Social Responsibility (CSR) activities as Chairman of the Tata Council for Community Initiatives (TCCI)," the statement said.
His appointment is with immediate effect and in his new role, he will report directly to Mistry, it added.
Rajan, 44, has been with the Tata group for over 18 years, after starting his career as a Tata Administrative Service (TAS) Officer.
Prior to the latest appointment, Rajan was Head of Private Equity Advisory at Tata Capital Ltd and Managing Partner of the advisory team of the Tata Opportunities Fund.
He had also been assigned to the office of the then Chairman of Tata Sons, Ratan Tata, in 1996, where he served for twelve years, including as Vice-President - Chairman's Office at Tata Sons.
An IIT Delhi graduate, he also holds Masters and Doctorate in International Relations from Oxford University.