Trump, making the first live video remarks to attendees by a sitting president, said he was honored to be part of the rally, which he described as made up of people from many backgrounds and places who are there for "one beautiful cause. To build a society where life is celebrated, protected and cherished."
"The march for life is a movement born out of love ... you love every child born and unborn because you believe that every life is sacred, that every child is a precious gift from God," Trump said.
Trump said the United States has some of the most permissive abortion laws of anywhere in the world, and is one of seven countries to allow elective late-term abortions.
"Under my administration, we will always defend the very first right in the Declaration of Independence, and that is the right to life," Trump said.
In a statement Friday, abortion-rights group NARAL Pro-Choice America said most Americans favor elective abortion rights.
"Every year, the 'March for Life' is a reminder of how a small group of individuals remains obsessed with shaming women for our personal decisions and controlling our lives," organization president Ilyse Hogue said. "The thousands here today are so dramatically out of line with mainstream Americans' values of dignity, compassion, and self-determination. "
Trump said he supports the House of Representatives' Pain-Capable Unborn Child Protection Act, a bill that would make it a crime for any person to perform an abortion if the fetus is 20 weeks along or more. The bill would allow for exceptions if an abortion is needed to save the life of the pregnant woman or if rape or incest caused the pregnancy.
"I call upon the Senate to pass this important law and send it to my desk for signing," he said.
Trump mentioned other efforts he is pursuing, including this week's creation of a division at the Department of Health and Human Services that will look into complaints from healthcare workers who say they have faced religious or moral discrimination at work.
"We are protecting the sanctity of life and the family as the foundation of our society," Trump said.
"I believe with all of my heart, with your continued dedication and compassion, with a pro-life majority in Congress, with President Donald Trump in the White House, and with God's help, we will restore the sanctity of life to the center of American law," Pence said.
House Speaker Paul Ryan, R-Wis., also addressed marchers when they rallied east of the Washington Monument, saying he's grateful to have a pro-life president in the White House.
"One thing that gets lost is how compassionate the pro-life movement is," he said. "To help women who have gone through the pain of abortion, to help single mothers, to give them resources through thousands of pregnancy centers: this is the face of the pro-life movement."
Among those attending the march were a group of Franciscan priests, who carried banners reading, "Keeping families together in pro-life! Keep God's dream alive!"
Meanwhile, a smaller group of pro-life advocates gathered outside the Smithsonian Air and Space Museum to protest Trump's speech. They said they support the belief that life begins at conception based on scientific research.
"Let's put some secular, pro-life, bad-ass feminists up front," Destiny Herndon-De La Rosa said.