Speaking via video feed to pro-life supporters in Washington for the March for Life rally, he said: "Under my administration, we will always defend the very first right in the Declaration of Independence and that is the right to life.
"Every unborn child is a precious gift from God."
He praised the thousands of anti-abortion activists gathered on the National Mall for having "such big hearts and tireless devotion to make sure parents have the support they need to choose life".
"You're living witnesses of this year's March for Life theme, 'Love Saves Lives," he added.
Trump also criticised the controversial later term abortion procedure, calling them "wrong" and a practice that "has to change."
"That is why we march, that is why we pray, that is why we declare that America's future will be filled with goodness, peace, joy, dignity and life for every child of God," he said.
US Vice President Mike Pence, who addressed last year's March for Life in person, introduced the president as the "most pro-life president in American history".
Meanwhile, dozens of celebrities have joined hundreds of thousands of people for women's marches across America in protest against President Trump and sexual assault.
Adele, Mila Kunis, Cameron Diaz and Jennifer Lawrence were among those who took part.
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