The firm already employs about 1,500 staff at its office in the Titanic Quarter.
The office support jobs will pay salaries of about £35,000.
First Minister Peter Robinson said: "This announcement by one of the most prestigious financial companies in the world reflects the firm's confidence in its Belfast operations."
He added: "The financial services sector is at the heart of the economy and the decisions and transactions that take place in Belfast will impact across the globe."
Deputy First Minister Martin McGuinness said: "Citi's presence here over the past 10 years has played a major role in developing the financial services sector, both in providing jobs and training.
"This reinvestment will create much needed highly-skilled jobs, and also increase the skills base of the financial services sector by introducing specialist investment banking functions."
Invest NI is offering grants of about £6m.
The firm first invested in Northern Ireland in 2004.
Its Belfast operation provides support such as legal and technology services to company operations across Europe, the Middle East and Africa.
James Bardrick of Citi said: "The success of our current operations in Belfast was an important factor behind this latest decision to create another 600 jobs.
"The supply of talent in Northern Ireland, particularly at graduate level, is impressive and, coupled with the support from Invest NI and the Department of Employment and Learning, it makes this region highly attractive to a global company like Citi seeking the best location for growth."
The global financial services company Citi is creating 600 new jobs in Belfast, in an investment worth £54m.