Bolsonaro is suffering an "intestinal subocclusion," a statement from the Vila Nova Star Hospital in São Paulo said. It's the latest medical issue linked to a 2018 incident in which Bolsonaro was stabbed on the campaign trail.
By Monday evening, the president was showing "clinical improvement," according to the hospital, who said in a statement that he had been able to take a "short walk" around the hallway.
There is no definitive evaluation as to whether the president will need surgery, it said.
Bolsonaro started feeling ill after lunch on Sunday and was admitted to hospital at 3 a.m. local time (1 a.m. ET), he wrote in a tweet.
The president said he had been given a gastric probe and more tests will be carried out to determine whether he needs surgery.
Bolsonaro was previously admitted to the hospital in July 2021 when physician Antonio Luiz Macedo found an intestinal obstruction.
Macedo also operated on Bolsonaro after he was stabbed during a campaign rally in September 2018. Bolsonaro underwent surgery In January 2019 to remove a colostomy bag fitted after the stabbing.
"It's the second hospitalization with the same symptoms, the result of the stabbing (September 6, 2018) and four major surgeries," wrote Bolsonaro in a second tweet, adding that Macedo had been traveling but would arrive at the hospital at 3 p.m. local time (1 p.m. ET).
Macedo told CNN affiliate CNN Brasil on Monday that he does not think that Bolsonaro will need surgery, based off of initial diagnostic reports that were sent to him.
"I am still going to examine him. My hypothesis, based on what I know about the president, is that there will be no surgery. But I need to do an abdominal evaluation in order to have a concrete conclusion," Macedo said, adding that he would arrive back to Brazil on Tuesday.
Bolsonaro is preparing to run for reelection in October, but opinion polls show his main rival, former president Luiz Inacio Lula da Silva, with a commanding lead.
The populist Bolsonaro has been heavily criticized for his handling of the pandemic, which has left Brazil as one of the world's worst-hit countries. The President has dismissed Covid-19 as a "little flu" and has refused to get vaccinated.
The nation has suffered from a slow vaccine rollout and staunch resistance to containment measures by Bolsonaro's government.
However in August 2021 the president refused to countenance defeat at the ballot box.
"I have three alternatives for my future: being arrested, killed or victory," said Bolsonaro, who has repeatedly raised doubts about his country's electoral system.