Fleming (pictured), from the US, replaces the UK's Alison Smale in the position after two years.
For the past 10 years Fleming has been head of global communications and spokesperson for the high commissioner for the Office of the United Nations High Commissioner for Refugees (UNHCR) in Geneva.
The UN said that during her time at UNHCR, Fleming led global media outreach campaigns, social media engagement and a multimedia news service serving audiences ranging from media, public, donors, governments and refugees themselves.
In addition to that role, from 2016 to 2017 she served as senior advisor and spokesperson on the incoming United Nations secretary general António Guterres’s transition team.
Before the UNHCR, the former journalist was spokesperson and head of media and outreach at the International Atomic Energy Agency.
Her earlier roles included heading the press and public information team at the Organisation for Security and Co-operation in Europe (OSCE), and being a public affairs specialist at Radio Free Europe/Radio Liberty in Munich.
In a statement, the UN said: "Ms Fleming brings to the position over 25 years of strategic vision, innovative management and communications expertise in multilateral organisations spanning the fields of human rights, humanitarian action, conflict prevention, peace-building, free media and nuclear non-proliferation, safety and security."
Fleming tweeted about her new role following the announcement:
I am honored to be appointed by @antonioguterres as the next @UN Under-Secretary-General for Global Communications.— Melissa Fleming (@melissarfleming) August 1, 2019
Communicating the ideals and values of the United Nations has never been more urgent.
I will remain forever committed to @Refugees.