The woman, who is from the Femen protest group, grabbed the waxwork crotch while shouting in English and Spanish.
"Grab patriarchy by the balls," she yelled. The phrase was also written on her back.
The statue, dressed in a suit and red tie, was being unveiled at the Madrid Wax Museum ahead of president-elect Trump's inauguration on Friday.
An employee of the museum tried to cover up her bare chest with a jacket.She wrestled briefly with an older man who tried to push her out and then the protester was led away.
The woman's name hasn't been revealed.
Femen, the group she is thought to belong to, is known for its high-profile protests often characterised by women going topless.
Donald Trump has been criticised in the past for his attitude towards women.A recording of him talking about grabbing women was released in the run-up to the US election last year.