US president-elect Barack Obama is resigning his Senate seat effective from Sunday, after serving less than a single six-year term following a lightning fast political ascent.
"It has been one of the highest honours and privileges of my life to have served the people of Illinois in the United States Senate," Obama said in a statement.
"In a state that represents the crossroads of a nation, I have met so many men and women who've taken different journeys, but hold common hopes for their children's future.
"It is these Illinois families and their stories that will stay with me as I leave the United States Senate and begin the hard task of fulfilling the simple hopes and common dreams of all Americans as our nation's next President."
Obama was elected in 2004 and spent only two years in the chamber before hitting the presidential campaign trail, culminating in his historic election victory last week over Republican John McCain.
It will now be up to the Democratic governor of Illinois Rod Blagojevich to name a replacement for Obama.
One of the favourites is Jesse Jackson Jr, son of the US civil rights leader and former presidential candidate Jesse Jackson.
The name of Valerie Jarrett, a close associate of the president-elect, has also been mentioned, but she has ruled herself out of the running, the Chicago Sun Times reported.