Defense Secretary Leon Panetta will make the formal announcement on Thursday.
"This policy change will initiate a process whereby the services will develop plans to implement this decision, which was made by the secretary of defense upon the recommendation of the Joint Chiefs of Staff," a senior defense official said.
Since the fighting began in Iraq and Afghanistan, nearly 300,000 women have served alongside men. Of the more than 6,600 who have been killed, 152 have been women.
Panetta's decision overturns a 1994 rule barring women from being assigned to smaller ground combat units and could open more than 230,000 jobs, many in Army and Marine infantry units, to women.
"I think America's daughters are just as capable of defending liberty and freedom as their sons are," said Ill. Rep. Tammy Duckworth, who was a helicopter pilot in Iraq and lost both her legs in combat.
Last February, the Defense Department opened up 14,500 positions, mostly in the Army, to women that had previously been limited to men, and lifted a rule that prohibited women from living with combat units. Panetta had also directed the services to examine ways to open more combat roles to women.
A senior military official says the services will develop plans for allowing women to seek the combat positions. Sources say the move would not take effect immediately, instead giving the individual branches of the military two years to argue that some roles should be limited to men only.Assessments for others, such as special operations forces, including Navy SEALS and the Army's Delta Force, may take longer.
Congress will have to be notified of each job that opens up before it can take effect. Some inside the services and on Capitol Hill say changes like this won't make ground troops more effective.
"To have women serving in infantry though could impair the mission and task of those units, and that's been proven in study after study ... I mean it's nature, upper body strength and physical movements and speed and endurance and so forth," said Ark. Rep. Tom Cotton.
Nancy Duff Campbell of the National Women's Law Center called the decision historic and that the change "opens the doors of opportunity to all women in the Armed Forces and eliminates the last vestige of government-sanctioned sex discrimination in the United States. Now if the best person for the job is a woman, she will no longer be barred from that job simply because of her gender."
This move by Panetta will be one of the last significant policy decisions he makes. He is expected to leave in mid-February, and it's not clear where former Sen. Chuck Hagel, Panetta's nominated replacement, stands.
Military chiefs will give their initial implementation plans to Panetta by May 15.
The Associated Press contributed to this report.
