In a report released Thursday, the rights group says the city was occupied from December 10, 2025, to January 17, 2026.
HRW said it documented more than 50 summary executions, at least eight rapes and 12 enforced disappearances, along with gunfire targeting civilians as they tried to flee.
The report is based on more than 120 interviews carried out in March and April this year.
The group further said M23 fighters and Rwandan forces went door to door, targeting men and boys accused of links to the Wazalendo militias.
One witness quoted in the report said four members of his family were shot as they tried to escape. He described the scene as chaos.
Human Rights Watch also says survivors of sexual violence had almost no access to medical care during the occupation, including treatment to prevent HIV infection.