Lisa P. Jackson is stepping down as administrator of the Environmental Protection Agency after a four-year tenure that began with high hopes of sweeping action to address climate change and other environmental ills but ended with a series of rear-guard actions to defend the agency against challenges from industry, Republicans in Congress and, at times, the Obama White House.

Ms. Jackson, 50, told President Obama shortly after his re-election in November that she wanted to leave the administration early next year. She informed the E.P.A. staff of her decision on Thursday morning and issued a brief statement saying that she was confident “the ship is sailing in the right direction.”

She has not said what she intends to do after leaving government, and no successor was immediately named, although it is expected that Robert Perciasepe, the E.P.A. deputy administrator, will take over at least temporarily.

Ms. Jackson’s departure comes as many in the environmental movement are questioning Mr. Obama’s commitment to dealing with climate change and other environmental problems. After his re-election, and a campaign in which global warming was barely mentioned by either candidate, Mr. Obama said that his first priority would be jobs and the economy and that he intended only to foster a “conversation” on climate change in the coming months.