"We're not going anywhere," Obama told a memorial service at Missouri Southern State University Sunday. "We will be with you every step of the way."
The massive tornado, which killed 142 people in this town of 50,000, was one of the worst ever in the United States.
Officials said late Sunday that at least 43 people remained missing, down from a list of 232 missing persons that was released on May 26.
The president's motorcade drove through some of the hardest-hit areas, where many homes had been destroyed by the 200 miles per hour (300 kilometer per hour) winds.
After the tour, Obama called the disaster "a national tragedy," and promised there would be "a national response."
At the memorial service, Obama recalled stories of heroism in Joplin, speaking of pizza shop manager Christopher Lucas, a father of two who ushered everyone into the freezer as the tornado approached.
The freezer door wouldn't close from the inside, so Lucas found rope and closed it from the outside.
"Tying a piece of bungee cord to the handle outside, wrapping the other end around his arm, holding the door closed with all his might," Obama said. "And Christopher held it as long as he could. Until he was pulled away by the incredible force of the storm.
"He died saving more than a dozen people in that freezer," he said. "You see, there are heroes all around us all the time."
At the memorial service, Missouri Governor Jay Nixon also pledged to rebuild.
"The people of Missouri were born for this mission," Nixon said. "We are famously stubborn and self-reliant, practical, impatient. No storm, no fire, no flood can turn us from our task.
"We can and we will heal. We've already begun," Nixon continued. "By God's grace, we will restore this community."
Crews continued searching for the missing, seven days after the tornado tore apart everything it touched along a path four miles (six kilometers) long.
The governor said officials are working "24 hours a day" to locate the missing and identify the deceased. He said that the battered condition of some of the bodies means that DNA tests have been needed to identify the remains.
State officials are cross-checking names of the missing with hospitals, and are working with cell phone service providers to determine if anyone has used their phone since being added to the list.
After releasing lower updated figures of the missing, the Missouri Department of Public Safety said there was "steady progress" in the effort, but added that the "objective continues to be reducing that number to zero, to help ease the anxiety of concerned loved ones."
The twister, a massive funnel cloud that struck on May 22, ranked as one of the deadliest tornado to hit the United States since modern record-keeping began in 1950.
More than 8,000 structures in the town, including a major commercial area, were damaged or destroyed when the tornado packing winds over 200 miles (320 kilometers) per hour came roaring through with just a 24-minute warning.
Joplin spokeswoman Lynn Onstot said the city was slowly getting back on its feet, although the traditional Memorial Day weekend opening of Joplin's public pools has been postponed.
"Public transportation is back up and running, and trash is running as normal as possible," although not in the disaster areas, she added.
A total of 318 people are living in temporary shelters in Joplin, state officials said Saturday.