The search-engine giant is pledging $2 million (£1.6 million) of its own money, while the remaining $2 million (£1.6 million) is expected to be made up in donations from Googlers.
On Friday, Trump signed an executive order for "extreme vetting" that halted the US refugee program and blocked citizens of a number of Muslim-majority countries from entering the United States for at least 90 days - even people who already have visas and legal permits to live in the United States. The countries affected are Iran, Iraq, Libya, Somalia, Sudan, Syria, and Yemen.
"We're concerned about the impact of this order and any proposals that could impose restrictions on Googlers and their families, or that could create barriers to bringing great talent to the US," Google said in a statement seen by USA Today. "We'll continue to make our views on these issues known to leaders in Washington and elsewhere."
The fund - reported to be Google's largest crisis campaign ever - will be awarded to American Civil Liberties Union, Immigrant Legal Resource Center, International Rescue Committee and UNHCR.
The Google campaign was reportedly disclosed in a message sent by Google CEO Sundar Pichai. It was then confirmed by a Google spokeswoman late Sunday.
The announcement of the Google crisis fund came a day after Pichai slammed the effects of the directive on Google staff.
"We're upset about the impact of this order and any proposals that could impose restrictions on Googlers and their families, or that could create barriers to bringing great talent to the US," he wrote.
"It's painful to see the personal cost of this executive order on our colleagues."
Separately, Uber is creating a $3 million (£2.4 million) legal defence fund to help drivers on its platform with immigration issues.