Herbert Spencer Hadley (February 20, 1872 – December 1, 1927) was an American lawyer and a Republican Party politician from St. Louis, Missouri.

As Attorney General, he successfully prosecuted Standard Oil Company for violating Missouri antitrust law.

Entering the 1912 Republican convention, the Roosevelt and Taft forces seemed evenly matched, and Hadley was seen as a possible compromise candidate.

In this position, Hadley developed a reputation for vigorous prosecution, including an investigation of jury tampering in the civil courts and a campaign against public gambling.

As attorney general, he prosecuted successful cases against Standard Oil Company, railroads, several trusts, and St. Louis racetrack gamblers.

This, combined with Hadley's success with the Standard Oil Company suit and his record for reform, contributed to his 1908 election as governor of Missouri.

Many of Hadley's recommendations for change in other government sectors, such as revenue and public service, were not supported by the Missouri General Assembly.