Investigations into the two US tech giants found they had been "collecting and analyzing" data on their users, and monitoring their use of websites and applications, the Personal Information Protection Commission said.
The data was used to "infer the users' interests or used for customized online advertisements", it said, adding that neither Google nor Meta had clearly informed South Korean users of this practice or obtained their consent in advance.
As a result, Google was fined 69.2 billion won ($49.7 million) and Meta 30.8 billion won ($22.1 million).
"It is the largest fine for the violation of the Personal Information Protection Act," the commission said in a statement.
Regulators said the majority of the users in South Korea -- 82 percent for Google and 98 percent for Meta -- had unknowingly allowed them to collect data on their online use.
"It can be said that the possibility and the risk of infringement of the rights of the users are high," the statement said.
Last year, South Korea fined Google nearly $180 million for abusing its dominance in the mobile operating systems and app markets, saying it was hampering market competition.
Giant US tech companies are regularly criticised for dominating markets by elbowing out rivals, with multiple governments globally seeking to rein them in.
The European Union has slammed Google with record antitrust penalties, and also gone after Apple and Microsoft.