The total price of the acquisition is Sg$4.9 billion ($3.6 billion).
As part of the deal, around 5,000 Citi employees are expected to transfer to UOB, the lenders said.
The sale is part of Citi’s plans to streamline its global operations and focus on institutional clients, rather than consumer banking.
The bank previously announced it was exiting consumer franchises in several markets across the Asia-Pacific and elsewhere.
Mark Mason, Citi’s chief financial officer, said that “the sale of these four consumer markets, along with our previously announced transactions, demonstrate our sense of urgency to execute our strategic refresh”.
Wee Ee Cheong, UOB’s deputy chairman and chief executive officer, said the bank “believes in Southeast Asia’s long-term potential and we have been disciplined, selective and patient in seeking the right opportunities to grow”.
The transaction is expected to be completed between mid-2022 and early 2024 depending on the progress of the regulatory approval process, Citi said.
Indonesia, Malaysia, Thailand and Vietnam are among Southeast Asia’s biggest markets, with a combined population of more than 470 million people and home to a rapidly growing middle class.