Mohibullah, 46, chair of the Arakan Rohingya Society for Peace and Human Rights (ARSPH), was shot and killed by unidentified gunmen in Kutupalong camp in Cox's Bazar, Bangladesh, UN News reported.
Bachelet described Mohibullah as "an exceptional human rights defender, who despite the risks that his work entailed, nevertheless continued defending the rights of his people."
The UN rights chief said that it is heartbreaking that a person who spent his life fighting to ensure that the violations committed against the Rohingya people were known world-wide has been murdered in this way.
Bachelet said Mohibullah's words "were very powerful and highlighted the terrible situation of the Rohingya and today, four years later, they echo as a reminder that Rohingya are still waiting for justice and still waiting to return home."
His death highlights the precarious situation of the Rohingya in both countries, she added, emphasising "we need to do much more to help this persecuted community, both in Bangladesh and in Myanmar."
On Friday, Meenakshi Ganguly, South Asia director at Human Rights Watch (HRW) had said Mohibullah was a vital voice for the community of Rohingya who had already suffered unimaginable loss and pain when they arrived as refugees in Bangladesh.
"He always defended the rights of the Rohingya to safe and dignified returns and to have a say in the decisions concerning their lives and future. His killing is a stark demonstration of the risks faced by those in the camps who speak up for freedom and against violence."
Meanwhile, the situation of roughly 600,000 Rohingya in Myanmar's Rakhine State remains dire, with many still confined in camps, and reports of alleged violations including unlawful killings, arbitrary arrest and detention, and high levels of extortion, UN News reported.