The New York billionaire donated $25,000 to Florida Attorney General Pam Bondi's campaign in 2013, as her office was considering whether to open an investigation into fraud allegations against Trump University.
Bondi's office did not open an investigation afterwards, Xinhua news agency reported.
"We have been concerned that the Trump Foundation may have engaged in some impropriety from that point of view," Schneiderman told CNN's "The Lead" programme on Tuesday, calling himself a "regulator of nonprofits in New York state".
"We have been looking into the Trump Foundation to make sure it's complying with the laws governing charities in New York," Schneiderman said.
Yahoo News reported last week that the Trump charity donated $100,000 to a conservative group that was suing Schneiderman as he was probing Trump University.
In a Tuesday campaign rally, US President Barack Obama, speaking on behalf of Democratic nominee Hillary Clinton, made a mockery of the Trump Foundation, highlighting reports that Trump used foundation money to buy a 6-foot (about 183 cm) portrait of himself.
Also on Tuesday, Democratic members of the House Judiciary Committee wrote a letter to Florida's Attorney General Loretta Lynchare, seeking a Justice Department investigation of the Trump Foundation over its donation to her.
"These payments may have influenced Bondi's official decision not to participate in litigation against Trump," the letter said, citing anti-bribery laws in their request.
The Trump campaign has repeatedly declined to provide evidence to show that Trump has donated as much to charity as he claims.
Defending his running mate, Republican vice presidential nominee Mike Pence said on Monday that Trump has donated millions of dollars in charity.
"Anyone who knows anything about Donald Trump and his career knows that this is a man who's given away tens of millions of dollars to charitable causes throughout the course of his business life -- he's been incredibly generous," Pence said, though offering no evidence.