"The Securities and Exchange Commission is a remarkable agency," Gensler said in a statement on Thursday. "The staff and the commission are deeply mission-driven, focused on protecting investors, facilitating capital formation and ensuring that the markets work for investors and issuers alike."

His departure will leave the SEC in the hands of an acting chair who's expected to be either Mark Uyeda or Hester Peirce — both Republican commissioners. 

Gensler, a self-described "markets guy" appointed by President Joe Biden in 2021, has pursed an aggressive agenda highlighted by climate-risk disclosures, stock-trading reforms and crackdowns on crypto scofflaws. Some of his regulations will leave a lasting imprint on finance. Others have been stymied in conservative courts. The Trump administration's coming pick for SEC chair could try to further unwind Gensler's signature rules and take a more crypto-friendly approach to enforcement.

Gensler's policy achievements include speeding up the settlement time for US stock trades and a new regulation that will result in trillions of dollars more in US Treasury market transactions to be centrally cleared each day. Corporate insiders also face stricter disclosures and rules for stock sales.

The next chair may reverse the focus on regulating hedge funds and private equity through stronger disclosures, and look for ways to quash the agency's climate-risk disclosure rules. Many of them already have been challenged in courts. Trump's SEC will probably ease enforcement against brokers, banks and hedge funds for using third-party messaging apps to communicate.

The digital-asset industry could experience a sea change in SEC policy. Gensler doggedly pursued crypto fraudsters, as well as companies like Coinbase Global Inc. and proprietary trading behemoth DRW Holdings for failing to register with the agency. The industry had pushed back hard, saying he had provided no real way for the nascent asset class to fit within decades-old structures.