There are no laws in Afghanistan that specifically prohibit sexual harassment or protect victims, the US-based rights body said.
Government institutions lack effective policies to prevent and punish sexual harassment, Human Rights Watch said.
On Oct 5, President Ashraf Ghani described levels of sexual harassment in schools as "shocking".
He ordered the education ministry to report every incident of sexual harassment in schools to enable action against the guilty, and directed ministries to develop a plan to counter sexual harassment in educational institutions.
"President Ghani's recognition of sexual harassment in Afghanistan as 'shocking' is spot on," said Heather Barr of Human Rights Watch.
"The Afghan government should promptly enact a law against sexual harassment and ensure that every government institution develops and implements an anti-sexual harassment police."
Sexual harassment is a major problem in Afghanistan, where women and girls have had to struggle to regain their rights after being completely shut out of education and employment during Taliban rule until their ouster in 2001.
There has been significant progress in improving girls' access to education and integrating women into the Afghan parliament and civil service.
But unchecked sexual harassment has been a significant obstacle to women's employment and participation in public life.
Harassment on the street was a daily experience for women and girls, and women who have sought help from the police in response to harassment and even threats have received no assistance, Human Rights Watch said.