Facebook was fined €110 million (£100 million) by the European Commission for providing misleading information to antitrust regulators when seeking approval to buy WhatsApp in 2014.
Margrethe Vestager, the EU's commissioner in charge of competition policy, said: "Today's decision sends a clear signal to companies that they must comply with all aspects of EU merger rules, including the obligation to provide correct information."
"And it imposes a proportionate and deterrent fine on Facebook," she said. "The Commission must be able to take decisions about mergers' effects on competition in full knowledge of accurate facts."
The Commission said Facebook claimed it would not be able to match Facebook users' accounts and WhatsApp users' accounts automatically.
"The Commission has found that, contrary to Facebook's statements in the 2014 merger review process, the technical possibility of automatically matching Facebook and WhatsApp users' identities already existed in 2014, and that Facebook staff were aware of such a possibility," the EU body said.
"We've acted in good faith since our very first interactions with the Commission and we've sought to provide accurate information at every turn," a Facebook spokesperson said. "The errors we made in our 2014 filings were not intentional and the Commission has confirmed that they did not impact the outcome of the merger review."
The sanction comes a day after French data regulators fined Facebook €150,000 for failing to prevent advertisers from accessing users' data.
The Commission's fine is the first of its kind under the 2004 Merger Regulation.