"Allowing the worst human rights violators to escape prosecution puts all Yemenis at risk of future abuses," she said in a statement Wednesday.
Former Yemeni President Ali Abdullah Saleh stepped down in February 2012. The terms of his resignation, brokered with the help of the region's Gulf Cooperation Council, gives him immunity from alleged crimes committed during his 22 years in office.
Human Rights Watch said Yemeni legislators should endorse a proposal to wipe that law from the books as part of a comprehensive dialogue aimed at national reconciliation. Whitson said amnesty runs counter to Yemen's international obligations to bring justice to those leaders suspected of human rights abuses.
"If the government doesn't take firm measures to end the legacy of impunity, it will only signal that nothing has really changed in Yemen," she said in a statement.
There was no information on the amnesty law published Wednesday by Yemen's official Saba News Agency.