The White House called the visit "disgusting" and said it showed that Democrats side with "an illegal alien MS-13 terrorist" while President Trump stands with law-abiding Americans.
Mr Ábrego García's lawyers deny he has any gang affiliation and maintain he has never been charged with, nor convicted of any crime.
Before the meeting, the senator said he was stopped by armed guards on his way to Cecot, the maximum-security prison where Mr Ábrego García has been detained.
Van Hollen arrived in the country on Wednesday hoping to secure the release of Mr Ábrego García, who had been living in Maryland.
The senator did not offer an update on Mr Ábrego García's status in his social media posts, but said more information would be released upon his return to the US.
Mr Ábrego García's wife celebrated the news and said her "prayers have been answered".
She said her family still has many questions and will continue fighting for his release.
During his trip, Van Hollen said he also met with the country's vice-president and asked that they open the doors so Mr Ábrego García could leave the prison, a request he says was rejected.
On X, El Salvador's president reposted photos of the senator meeting Mr Ábrego García and appeared to poke fun at social media speculation that the inmate had died in custody.
President Bukele commented that Mr Ábrego García had "miraculously risen from the 'death camps' & 'torture'" in the "tropical paradise of El Salvador".
"Now that he's been confirmed healthy, he gets the honor of staying in El Salvador's custody," the president added.
Mr Ábrego García was living in Maryland before he was deported on 15 March with scores of Salvadorans and Venezuelans to the Center for the Confinement of Terrorism (Cecot) in El Salvador.
Maryland Judge Paula Xinis ruled that Mr Ábrego García's removal from the country breached a 2019 court order that had granted him legal protection from deportation.