The board of the Boy Scouts of America (BSA) on Wednesday unanimously decided to allow girls to enrol in Cub Scouts and eventually earn the honor of becoming Eagle Scouts.
The 100-year-old BSA said the historic shift was a result of years of requests from families and girls.
"We believe it is critical to evolve how our programs meet the needs of families interested in positive and lifelong experiences for their children," Boy Scouts Chief Executive Michael Surbaugh said in a statement.
The families can choose to sign up their girls for the 7- to 10-year-old Cub Scouts starting 2018.
Small, community-level "dens" will be single-gender — all boys or all girls. Larger "packs," which comprise several dens, can choose whether to include dens of each gender or not, the BSA said.
The move has prompted a backlash, most notably from the Girl Scouts of the USA, which said girls thrive in a girl-only environment, and that "we are girl experts."
"Girl Scouts remains committed to and believes strongly in the importance of the all-girl, girl-led and girl-friendly environment that Girl Scouts provides, which creates a necessary safe space for girls to learn and thrive," the organization said in a statement, without mentioning the BSA.
In August, Girl Scout President Kathy Hopinkah Hannan accused the BSA of running a "covert campaign" to recruit girls to increase its declining membership.
BSA member count has dropped by a third since 2000 and now stands at 2.3 million. But the organization insisted that the move to welcome girls was in response to the needs of families, not related to declining membership.
Among the hundreds who criticized the move on social media was US President Donald Trump's eldest son Donald Trump Jr.
In January, the Boy Scouts opted to allow transgender boys to join the group.