A global watchdog, the Financial Action Task Force (FATF), on Friday said it had placed South Africa on its "grey list" for increased monitoring over deficiencies in combating "money laundering, terrorist financing, and proliferation financing."
The FATF sets standards for more than 200 countries and jurisdictions on cross-border financial crime -- a system intended to encourage national authorities improve their performance.
"The grey listing is an opportunity for us to tighten our controls and improve our response to organized crime", Ramaphosa said in his weekly newsletter.
"The situation is concerning but less dire than some people suggest," he added.
"The fundamentals are in place and we know what we need to do to get off the grey list," Ramaphosa said, stressing he was "determined to do this as quickly as possible".
South Africa's state sector was devastated by corruption under former president Jacob Zuma, which Ramaphosa vowed to clean up when he took office five years ago.
The radical leftist opposition party, the Economic Freedom Fighters (EFF), reacted strongly to the grey listing, lashing Ramaphosa as "a criminal who is leading a criminal state".
Ramaphosa is "part of the reasons why South Africa has been grey listed," it said on Friday.
South Africa and Nigeria were the only new entries to the "grey list," whose 23 members include the Cayman Islands, Democratic Republic of Congo, the Philippines, Syria and Yemen.
The listing is "further indictment on the ANC national government's failure to deal decisively with financial crime," John Steenhuisen, the head of the leading opposition party, the Democratic Alliance (DA), said on Twitter.
The listing is "hardly surprising when our sitting President stashes millions of US dollars in his own couch," Steenhuisen said, referring to the alleged coverup of a massive theft at Ramaphosa's ranch.
Corruption in South Africa has fueled a decline in support for the ruling African National Congress (ANC) ahead of the 2024 elections - a fall that that may cost Ramaphosa the presidency.