"The red MAGA hat is the new white hood," Milano, 46, tweeted Sunday.
"Without white boys being able to empathize with other people, humanity will continue to destroy itself. #FirstThoughtsWhenIWakeUp," she added.
Milano had previously posted a video of Nick Sandmann, a junior at Covington Catholic High School, and Native American activist Nathan Phillips.
Milano's post of the video came before a longer video emerged showing that Sandmann and his fellow students, who had participated in the March for Life earlier Friday, may have faced harassment of their own from other groups of protesters.
Sandmann, a junior at Covington Catholic High School in Kentucky, released a statement claiming he was targeted for harassment.
I am "mortified that so many people have come to believe something that did not happen — that students from my school were chanting or acting in a racist fashion toward African-Americans or Native Americans. I did not do that, do not have hateful feelings in my heart and did not witness any of my classmates doing that," he said.
He added that other protesters called him and his classmates "'racists,' 'bigots,' 'white crackers,' 'f—ts,' and 'incest kids.' They also taunted an African-American student from my school by telling him that we would 'harvest his organs.'"
The former "Charmed" star has been extremely open about her political beliefs.
In November, she announced she refused to speak at the 2019 Women's March unless its founders condemned Louis Farrakhan.
EDITOR'S NOTE: A previous version of this report identified Phillips as a Vietnam War veteran. That has since been found not to be the case.