Nambiar is currently Ban's Chief of Staff, a position he has held since 2007.
"Nambiar expressed (his) wish to step down, so as to allow me to compose a new team of senior managers for his second term as UN Chief," Ban told reporters announcing the appointment.
Nambiar will move to serve as Ban's "Special Advisor on Myanmar at an appropriate time, following the transition in my Executive Office," the UN Chief said, thanking the former Indian Permanent Representative to the UN for his "unfailing support, wise counsel, and dedication in handling the many challenges that have faced the organisation during my first term.
Nambiar previously served as India's ambassador to Pakistan, China and Afghanistan.
Ban said the United Nations has been playing a key role in furthering the democratisation process of Myanmar.
"Encouraged" by the recent release of political prisoners in the country, Ban said he is planning to visit Myanmar in the near future to have further discussions with the authorities.