Rumsfeld delivered the news to U.S. troops in the former Iraqi insurgent stronghold of Fallujah.
"At the recommendation of our military commanders and in consultation with our coalition partners and with the Iraqi government, President Bush has authorized an adjustment in U.S. combat brigades in Iraq from 17 to 15," Rumsfeld said.
This is the first time Rumsfeld has said the number of U.S. troops in Iraq will dip below what has been the baseline for months – 138,000.
The secretary did not reveal the exact size of the reduction but Pentagon officials say it could be as many as 7,000 combat troops, CBS News Correspondent Susan Roberts reports. It means two Army brigades scheduled for tours — one from Fort Riley, Kan., the other now in Kuwait — will no longer deploy to Iraq.
The cuts, set for spring, come on top of the planned return of 20,000 troops sent to Iraq to help with election security.
Further reductions will be considered "at some point in 2006," after the new Iraqi government is in place and is prepared to discuss the future U.S. military presence, Rumsfeld added.
In other recent developments:
The Bush administration has insisted for some time that U.S. troops can start coming home once Iraqi soldiers are able to take over the fight, but the president has been forced to aggressively defend his Iraq strategy amid slumping public support and increasing pressure from Congress for a withdrawal plan.
The American force peaked at 192,000 during the March 2003 invasion; the monthly low was 109,000 in January 2004.
Earlier in Baghdad, Rumsfeld, on an unannounced trip to Iraq, was asked by reporters whether he had decided to hold back the deployment of the 1st Brigade of the 1st Infantry Division from Fort Riley, Kansas, and the 2nd Brigade of the 1st Armored Division, now in Kuwait.
Rumsfeld would not answer directly, but then elaborated during his visit to Fallujah.
Upon his arrival in the Iraqi capital Thursday, Rumsfeld met with Gen. George Casey, the top U.S. commander in Iraq, and Zalmay Khalilzad, the U.S. ambassador to Baghdad.
This is Rumsfeld's 11th trip to Iraq since the war began.
For the first time since the insurgency took hold in Iraq in midsummer 2003, Rumsfeld spent the night in the country. He previously had made Iraq day trips but spent the night in other countries in the region.
The issue of troops reductions came up earlier during Rumsfeld's visit to Afghanistan.
"Well, we're not going to withdraw precipitously. We're going to finish the important work that's being done there," he responded.
During Rumsfeld's stop in Afghanistan, military officials said they were making good progress toward eliminating the Taliban resistance and al Qaeda terrorists who continue sporadical violence against U.S. troops.
But some officers said the hostile forces are making some gains by acquiring more advanced weaponry, such as armor-piercing munitions, and improving their training and organization.