WASHINGTON — A 17-year-old transgender youth, Leelah Alcorn, stunned her friends and a vast Internet audience in December when she threw herself in front of a tractor-trailer after writing in an online suicide note that religious therapists had tried to convert her back to being a boy.

In response, President Obama is calling for an end to such therapies aimed at “repairing” gay, lesbian and transgender youth. His decision on the issue is the latest example of his continuing embrace of gay rights.

In a statement that was posted on Wednesday evening alongside a WhiteHouse.gov petition begun in honor of Ms. Alcorn, Mr. Obama condemned the practice, sometimes called “conversion” or “reparative” therapy, which is supported by some socially conservative organizations and religious doctors.

The petition has received more than 120,000 signatures in three months.

“We share your concern about its potentially devastating effects on the lives of transgender as well as gay, lesbian, bisexual and queer youth,” the statement, written by Valerie Jarrett, a senior adviser to Mr. Obama, says. “As part of our dedication to protecting America’s youth, this administration supports efforts to ban the use of conversion therapy for minors.”