Tulsi Gabbard, the director of national intelligence and an advocate of a more restrained foreign policy, submitted a letter of resignation to President Trump on Friday, saying that she was stepping away to support her husband after he recently was diagnosed with a rare form of bone cancer.

The departure will bring an end to Ms. Gabbard’s rocky tenure overseeing the 18 U.S. intelligence agencies, during which she had been largely sidelined by the White House on significant national-security issues, including military operations in Iran and Venezuela.

Mr. Trump assembled a second administration last year with an unusual mix of foreign-policy hawks in one corner and critics of American entanglements overseas like Ms. Gabbard in the other. But Ms. Gabbard and her wing found themselves increasingly marginalized in recent months, and her departure marks the most significant exit yet from the coalition of restrainers.

In her letter Friday, Ms. Gabbard, 45, said she would remain in her post as the nation’s intelligence chief until June 30.

“My husband, Abraham, has recently been diagnosed with an extremely rare form of bone cancer,” Ms. Gabbard said in her resignation letter, a copy of which was released by her office. “He faces major challenges in the coming weeks and months. At this time, I must step away from public service to be by his side and fully support him through this battle.” Ms. Gabbard has been married to Abraham Williams, a cinematographer, since 2015.