The new Somali parliament, whose members were selected by traditional elders last month, will hold a secret ballot Monday to choose the next president.
"September 10 will be a historic day for Somalia as its parliament chooses a new president," said acting deputy State Department spokesman Patrick Ventrell.
"This is a time for parliament to act with courage, determination and integrity in conducting a fair and transparent election."
Somalia is trying to set up its first stable central government since the 1991 ouster of dictator Mohamed Siad Barre that sparked years of civil war.
At least a dozen candidates are expected to run for office, including outgoing president Sheikh Sharif Sheikh Ahmed, in power since 2009.
But bitter arguments have already broken out between the challengers, divided along Somalia's notoriously fractious clan lines.
"It is imperative that all candidates for president recognize the outcome of this process," Ventrell said, adding that the security forces must also respect the new administration and protect human rights.
"Our hope is that the winner as well as the new members of parliament will commit to an inclusive government dedicated to improving the lives of all Somalis," he added.
"We also urge that candidates who lose will do so gracefully, accept the result and refrain from inciting violence or encouraging others to distance themselves from Somalia's new federal governmental institutions."