The investigation will seek to determine if Google has violated the Missouri Merchandising Practices Act - its principal consumer-protection law - and other state antitrust laws, according to a statement.
He says the company hasn't yet received an investigative subpoena issued by Hawley's office.
Hawley said the Federal Trade Commission under former President Barack Obama "did not take any enforcement action against Google, did not press this forward and has essentially given them a free pass".
The Kansas City Star reported on Monday that Attorney General Josh Hawley has issued subpoenas to Google as part of the investigation.
Google didn't immediately respond to a request for comment.
Google has largely steered clear of antitrust problems in the U.S. That's not the case in Europe, where the company faces a fine of about $2.7 billion over the display of its shopping ads.
National regulators last probed Google in 2013, when the Federal Trade Commission (FTC) reached a settlement with the internet company.
Missouri is launching an investigation into whether Google has broken its consumer protection and antitrust laws.
Hawley noted Google has access to an estimated 70 percent of all card transactions in the United States, as well as online users' location, device information, cookie data, online queries and website history. "My office will not stand by and let private consumer information be jeopardized by industry giants, especially to pad their profits".