In an open letter written on Thursday, Tutu, 85, called on the 72-year-old leader to stop the ethnic cleansing and genocide of the Rohingya minority.
In the letter addressing his “dearly beloved younger sister,” he expressed “profound sadness about the plight of the Muslim minority” in Myanmar.
"My dear sister: If the political price of your ascension to the highest office in Myanmar is your silence, the price is surely too steep," the retired Catholic Bishop said to Suu Kyi, who is now serving as Myanmar's state counselor.
In the 2015 elections, her junta-backed party won a landslide victory, taking 86 percent of the seats in the Assembly of the Union, and she became head of the government.
“As we witness the unfolding horror we pray for you to be courageous and resilient again,” Tutu said, adding, “We pray for you to intervene in the escalating crisis and guide your people back towards the path of righteousness.”
Suu Kyi, is the daughter of assassinated Major General Aung San, who is considered the Father of the Nation. She won the Nobel Peace Prize in 1991.
She was a prominent political prisoner of the military junta ruling the country back then.
Many critics have called for her Nobel prize to be revoked, citing her silence over the persecution of the Rohingya.