"We are very happy to have Aya back home and it's a great honour to have her in the Oval Office, with her brother," Trump said, declining to answer questions about her case.
Hijazi was accompanied by her brother, Basel, as she met Trump in the Oval Office.
She was acquitted by a Cairo court on Sunday along with seven others who had worked with street children.
Hijazi, 30, and her Egyptian husband established the Belady foundation to aid street children in 2013, but were arrested in 2014.
She had been in custody for 33 months in violation of Egyptian law, which states that the maximum period for pretrial detention is 24 months.
US officials had raised Hijazi's case with Egypt soon after Trump took office on January 20, aides said.
Trump said he struck "no deal" for her release, but that he had raised the issue as he hosted Egyptian President Abdel Fattah el-Sisi at the White House at the beginning of this month.
"He was here, I said I really would appreciate it if you could look into this and let her out," Trump told The Associated Press. "I asked the government to let her out."
He made no public mention of her case at the time, and activists criticised the administration for glossing over Egypt's poor human rights record while focusing on trade and military cooperation.
Pressed on how Trump managed to gain her release when President Barack Obama had not, White House spokesman Sean Spicer said he would leave it to others "to look at the different strategies to see why the president was successful" and Obama was not.
Since toppling President Mohamed Morsi of the Muslim Brotherhood in mid-2013, Sisi's government has cracked down on the opposition, killing hundreds of Brotherhood supporters and jailing thousands. Liberal and secular activists have also been arrested.