Rubinstein, who has deep knowledge of the Middle East, has spent more than a year as the State Department's representative to Syria. He is fluent in Arabic and previously served in Jordan, Israel, Egypt and Iraq.
His nomination comes as leaders from G7 industrialized nations said on Monday they believed a window of opportunity for a political deal in Syria may be opening up with President Bashar Assad facing pressure on the battleground.
While Group of Seven sources said a breakthrough was not imminent, they said Assad looked increasingly on the defensive after setbacks for his army.Rubinstein was recently in Moscow to try to see whether a political deal could be forged on Syria. The Syrian army and allied militia have lost large areas of land in the past two months to insurgents, including the al-Qaida-linked Nusra Front and Islamic State.