"Lissu has suffered multiple gunshot wounds and is being treated at the main public hospital in Dodoma," Tumaini Makene, spokesman for the main opposition CHADEMA party, said.
CHADEMA said in a separate statement the incident took place at his Dodoma residence on Thursday afternoon.
"We strongly condemn this attack and are closely monitoring his condition," the statement said.
Lissu, who is also the president of the Tanganyika Law Society and opposition chief whip in parliament, has previously been arrested by police on several occasions and charged with incitement.
He was arrested in July and later released after having called Magufuli a "dictator".
The motive of the shooting is unclear.
Tanzania police's spokesman was not immediately available for comment.