The bank said it would cut the number of its branches to 134 from 190 over the next several months and enhance online services for mobile and tablet platforms.
"Korea is one of Citi's most important markets globally and these actions will ensure a long term annuity of sustainable growth for Citi's retail business in Korea," a Citi spokesman said.
The closures will lead to job cuts, but numbers haven't been decided yet, said a Citibank Korea official, who declined to be identified.
Citi is the latest foreign financial business reduce presence or pull out of South Korea in the face of increased competition with bigger local players such as Woori Bank and Shinhan Bank.
In July, HSBC Holdings PLC decided to exit retail banking and wealth management in South Korea. Standard Chartered PLC is seeking to sell its South Korean consumer-finance business and savings bank after last year posting a $1 billion write-down in the value of its Korean business.
Citibank Korea posted a net profit of 219 billion won ($208 million) last year, down 8.1% from a year earlier, mainly due to an interest-income decline and increased loan-loss reserves.
Citibank Korea said Tuesday its business in Korea now will be concentrated in six major cities, including Seoul. It will aim its online enhancements at the whole country.
Citigroup opened its first South Korean branch in 1967. It has conducted three rounds of voluntary retirements over the past few years, through which some 600 employees have left the company.