"It's important to know when it is not your time and to know how you can best serve your community and country," Gillibrand said in a video posted to social media.
Though she represented one of the nation's most high-profile states, Gillibrand's campaign struggled to gain traction from the start. She lagged in the polls upon her entrance to the race in March and surpassed the donor threshold required for participation in the first two rounds of debates well after several of her Senate colleagues in the race, as well as unheralded names like Andrew Yang.
Looking ahead to September's third debate, her campaign had said she was 20,000 contributors and two qualifying polls shy of the requirements needed by the deadline. As of Wednesday morning, she was still calling on supporters via social media to help her cross the donor threshold.
Gillibrand made women's issues the cornerstone of her campaign, releasing policy plans focused on paid family leave, reproductive rights and equal pay. Prior to her candidacy, she spoke out on sexual assault on college campuses and pushed to remove hearings concerning sexual assault in the military from the chain of command. She advocated for landmark legislation such as the repeal of Don't Ask, Don't Tell and the James Zadroga 9/11 Health and Compensation Act.
But it was her 2017 decision to call for the resignation of her Senate colleague, Al Franken, D-Minn., in the midst of allegations of inappropriate behavior, that became most associated with her candidacy. Many voters indicated that they thought the action deprived Franken of due process.
The senator often said on the campaign trail that fundraising efforts were hampered by donors who she said were aiming to punish her for speaking out against Franken.
"There's some Democratic donors who want to punish me for the behavior of Sen. Franken and hold me accountable for his decisions," she said in an interview with The Washington Post Live earlier this month.
Gillibrand often touted her congressional win in a "two-to-one Republican district" with "more cows than Democrats," but her record from those years on immigration and guns left her vulnerable to critics. Gillibrand, who once had an A rating from the NRA, said she changed her stance when she filled Clinton's Senate post and became aware of gun violence in urban parts of the state. She also apologized for her previous stance on immigration, which included opposition to amnesty for undocumented immigrants and closing the border.
In an interview published in The New York Times just prior to the release of her video, Gillibrand revealed that she would endorse another candidate in the primary, but has not yet made a final decision. She did say, however, that she believed a fellow woman could be the best choice.
"I think that women have a unique ability to bring people together and heal this country," she said, "I think a woman nominee would be inspiring and exciting."
Several of the presidential field's remaining 20 candidates shared well-wishes via Twitter to Gillibrand upon learning about her decision Thursday evening, including her close friend, Sen. Cory Booker, D-N.J.
"Kirsten, you are my sister and one of the most righteous fighters I know," Booker tweeted. "I'll miss our run-ins on the trail, but women, New Yorkers, and all Americans are lucky to have you resolutely at their sides."
Sen. Kamala Harris, D-Calif., called Gillibrand "a brave voice on some of the most critical issues facing our country today -- from childcare to sexual assault."
"She is a champion and I know she’s not done fighting for women and families everywhere," she added.
"She is a forceful voice against Donald Trump and all he represents, and I look forward to working with her to defeat him," said Sen. Bernie Sanders, I-Vt.
Sen. Elizabeth Warren, D-Mass., thanked Gillibrand for her commitment to women's rights.
"Your voice has been strong and clear, and your determination is always on display," said Warren. "I'm proud to keep fighting alongside you."
President Donald Trump also weighed in on the news, tweeting, "A sad day for the Democrats, Kirsten Gillibrand has dropped out of the Presidential Primary. I’m glad they never found out that she was the one I was really afraid of!"
He had called her a "lightweight," a "total flunky for Charles E. Schumer" and "very disloyal" in December 2017.