Doctor’s announcement comes as presidential candidate’s departure from 9/11 memorial raises questions about her health.
Hillary Clinton has been diagnosed with pneumonia by her personal doctor after the US Democratic presidential candidate fell ill at a 9/11 memorial.
The episode has renewed focus on her health less than two months before an election.
She was diagnosed on Friday, the doctor said, but her condition was not made public until Sunday afternoon.
Just hours earlier, a video was posted on social media, apparently showing Clinton stumbling and her knees buckling, before being helped by aides into a black van leaving the site of the September 11, 2001, attack in New York City.
She was taken to her daughter Chelsea’s home in the city and appeared on her own about two hours later, wearing sunglasses and telling reporters that she was "feeling great".
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The temperature in New York was about 27C, combined with high humidity.
Clinton wore a high-collared shirt and a dark suit during the memorial honouring the nearly 3,000 people killed in the 9/11 attacks.
Democratic Congressman Joe Crowley said that it was "stiflingly hot", and he himself had to leave the ceremony.
Clinton’s doctor, Dr Lisa Bardack, said in a statement that his patient had been experiencing a cough-related to allergies and that an examination on Friday showed it was pneumonia.
"She was put on antibiotics and advised to rest and modify her schedule. While at this morning’s event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely," Bardack said.
Clinton cancelled her campaign trip to California on Monday because of her diagnosis of pneumonia.
Al Jazeera’s Kimberly Halkett, reporting from Washington DC, said Clinton’s health would now become a "major issue" in the lead-up to the election, "elevated from the ranks of conspiracies to a legitimate campaign issue".
She noted that Sunday’s event was Clinton’s second health-related incident in a week.
Clinton’s speech at a campaign rally on Labour Day in Cleveland was interrupted by a coughing spell. During the speech, she quipped: "Every time I think about Trump I get allergic."
She then resumed her speech.
Republican rival Donald Trump and his supporters have been hinting at potential health issues for months, questioning Clinton’s stamina when she takes routine days off the campaign trail and reviving questions about a concussion she sustained in December 2012 after fainting.
Her doctor attributed that episode to a stomach virus and dehydration.
Clinton’s doctor reported that she is fully recovered from the concussion, which led to temporary double vision and discovery of a blood clot in a vein in the space between her brain and skull.
Clinton also has experienced deep vein thrombosis, a clot usually in the leg, and takes the blood thinner Coumadin to prevent new clots.
Trump attended the same event marking the 15th anniversary of the 9/11 attacks. Asked by a reporter about Clinton’s health incident, Trump said: "I don’t know anything."
Past presidential candidates have released much more detailed information about their health than either Trump, 70, or Clinton, 68.