Deputy Defense Secretary Ashton Carter will step down as the Pentagon's No. 2 official in December, according to the Pentagon.
Carter informed Defense Secretary Chuck Hagel that he has decided to leave the department on Dec. 6, the Defense Department chief said on Thursday.
"I will always be grateful that Ash was willing to stay on and serve as my deputy secretary," Hagel said.
"He possesses an unparalleled knowledge of every facet of America's defense enterprise, having worked directly and indirectly for eleven Secretaries of Defense over the course of his storied career," Hagel added.
Carter's resignation ends his four-plus-year stint as the department's second-in-command.
Carter, as well as former Under Secretary for Policy Michèle Flournoy, were the assumed front-runners to replace former Pentagon chief Leon Panetta when he left the department last November.
But President Obama eventually selected Hagel for the top job at the Pentagon, nominating Carter to be Hagel's deputy.
Carter's decision to leave the department was not prompted by pressure from White House or Pentagon officials, according to Pentagon Press Secretary George Little.
"The decision to depart the Pentagon later this year was Deputy Secretary Carter's and his alone," Little said in a statement Thursday.
Hagel and Carter have "a strong and effective working relationship and friendship that will continue for the next two months and beyond," Little added.
Senate Armed Services Committee member Lindsey Graham (R-S.C.) praised Carter's tenure at the Pentago in the face of the "most challenging times in the department’s history."
"Ash Carter was always there providing exceptional leadership at a time when it mattered. He will truly be missed," the South Carolina Republican said in a statement Thursday.
As the Pentagon's deputy chief, Carter spearheaded efforts to rein in costs and excessive spending in the department's expansive weapons development programs, an effort he began as the department's top weapons buyer under former Defense Secretary Robert Gates.
He also drafted the department's new strategy to deal with the massive, across-the-board budget cuts under the White House's sequestration plan.
Released in July, the plan considers shrinking the Army from 490,00 to between 380,000 and 450,000 troops to deal with the $500 billion in mandatory spending cuts.
The cuts began in March and would reduce Pentagon spending by $52 billion next year.
The Marine Corps would be slashed from 182,000 to between 150,00 to 175,000, and the number of aircraft carrier strike groups would be reduced from 11 to eight or nine, under that plan.
Another scenario outlined in that sequestration strategy would keep current troop numbers, but abandon vital weapons programs designed to replace several aging systems in the Pentagon’s arsenal.
At the time of his resignation, Carter was overseeing a Pentagon-wide review of the department's security measures at military installations, in the wake of September's mass shooting at the Navy's Washington, D.C., headquarters.
--This report was updated at 5:37 p.m.