"The need to secure our border is urgent," the US president said in a rare, nationally broadcast primetime speech, underscoring the importance of the issue.
"In coordination with governors, up to 6,000 Guard members will be deployed to our southern border" for up to one year, Mr Bush said.
The National Guard troops will assist border patrol forces in operating surveillance systems, analysing intelligence, installing fences and vehicle barriers and other tasks, he said.
Mr Bush denied he was "militarising" the border but said he wanted to use the troops to back up the Border Patrol while he added 6,000 officers over the next two years.
A US official said the president is asking Congress for $US1.9 billion ($A2.5 billion) in emergency funds to pay for beefing up border security.
Conservative concerns
The president said the Border Patrol would remain in the lead and the Guard would assist by operating surveillance systems, analysing intelligence, installing fences and vehicle barriers, building patrol roads and providing training.
He also emphasised that the new immigration enforcement duties would in no way interfere with other functions carried out by the National Guard.
"It is important for Americans to know that we have enough Guard forces to win the war on terror, respond to natural disasters, and help secure our border," Mr Bush said.
The president was trying to placate conservatives demanding tougher border enforcement. But to their chagrin, he also insisted on creating a guest-worker program.
Defending his proposal for a guest-worker program, Mr Bush said: "The reality is that there are many people on the other side of our border who will do anything to come to America to work and build a better life."
"I support a temporary worker program that would create a legal path for foreign workers to enter our country in an orderly way, for a limited period of time," he said.