Karen Hughes, a long-time adviser to US President George W Bush and the state department's public diplomacy chief, has said she will step down this year.
Her departure follows that of close Bush aides Karl Rove and Dan Bartlett. The trio from Texas are widely credited with Mr Bush's rise to the White House.
Ms Hughes has spent the past two years trying to improve the image of the US abroad, especially in the Middle East.
She was previously communications chief for Mr Bush when he was Texas governor.
She then became one of his closest advisers when he first arrived in the White House.
After a two-year spell back in Texas from 2002-04, she rejoined Mr Bush's team as an election planner, going on to become undersecretary of state for public diplomacy in 2005.
In that role, she was responsible for trying to polish the image of the US overseas in the wake of the Iraq war and the damaging Abu Ghraib prison abuse scandal.
National security
US Secretary of State Condoleezza Rice said she had accepted Ms Hughes's resignation "with a great deal of sadness but also a great deal of happiness for what she has achieved".
Ms Hughes was behind the setting up of "rapid response" public relations units abroad to handle news events.
She also ensured that more Arabic speaking officials were available for interview by Arabic media outlets.
However, polls suggest that the popularity of the US overseas remains low, affected by the aftermath of the Iraq war and the unresolved Israel-Palestinian conflict in particular.
Speaking after her resignation was announced, Ms Hughes said she felt she had fulfilled her mission "by transforming public diplomacy and making it a national security priority central to everything we do in government".
Ms Hughes intends to leave the state department by the end of the year, probably in mid-December.
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