Vaccinated Americans don’t need masks outdoors in small groups or when biking and running, the C.D.C. says.

The Centers for Disease Control and Prevention took a major step on Tuesday toward coaxing Americans into a post-pandemic world, relaxing the rules on mask wearing outdoors as coronavirus cases recede and people increasingly chafe against restrictions.

The mask guidance is modest and carefully written: Americans who are fully vaccinated against the coronavirus no longer need to wear a mask outdoors while walking, running, hiking or biking alone, or when in small gatherings, including with members of their own households. Masks are still necessary in crowded outdoor venues like sports stadiums, the C.D.C. said.

But President Biden hailed it as a landmark moment in the pandemic, wearing a mask as he approached the lectern on a warm spring day on the White House grounds — and pointedly keeping it off as he walked back into the White House when he was done.

“Go get the shot. It’s never been easier,” Mr. Biden said. “And once you’re fully vaccinated, you can go without a mask when you’re outside and away from big crowds.”

The C.D.C. stopped short of telling even fully vaccinated people that they could shed their masks outdoors altogether — citing the worrying risk that remains for transmitting the coronavirus, unknown vaccination levels among people in crowds and the still-high caseloads in some regions of the country. The guidance also cautioned even vaccinated people against going without masks in medium-size outdoor gatherings.

But even the C.D.C.’s director, Dr. Rochelle Walensky, emphasized a more expansive interpretation, telling reporters at a White House briefing, “We no longer feel that the vaccinated people require masks outdoors,” outside “large public venues, such as concerts, stadiums and things like that.”

On Capitol Hill, a group of Republican lawmakers who are also medical professionals released an advertisement on Tuesday encouraging vaccination, in which they appeared wearing white coats with stethoscopes draped around their necks. Senator Roger Marshall, a freshman Republican from Kansas and a medical doctor, told viewers that the reason to get vaccinated was simple: “So we can throw away our masks, and live life as free as before.”

Mr. Marshall, who organized the effort, said it was based on research conducted by Frank Luntz, a Republican pollster working to reduce vaccine hesitancy among conservatives. In an interview, Mr. Luntz said Mr. Biden’s announcement was a positive step, and could give people who are reluctant to get vaccinated a reason to get their shots.

For Mr. Biden, who will address Congress on Wednesday and mark his 100th day in office on Thursday, the C.D.C.’s announcement was a moment to bask in what he called the “stunning progress” Americans had made since he took office. Next week, he said, he will outline a plan “to get us to July 4 as our target date to get life in America closer to normal and begin to celebrate our independence from the virus.”

Americans have been whipsawed on the issue of mask wearing since the beginning of the pandemic, when top health officials said people did not need them — in part because of severe shortages of protective gear for health care workers on the front lines. Masks became the centerpiece of the culture wars that surrounded the pandemic, especially after President Donald J. Trump insisted that they were optional and that he would not wear one.