But Marko Sladojevic, one of Karadzic's advisers, told the AP news agency that the 64-year-old will not co-operate with the lawyer.
"Mr Harvey has absolutely no knowledge whatsoever about the case," he said, adding Karadzic believes he should have been allowed to make his own choice of lawyer.
"This affects his fair trial rights," he said.
Among the key charges Karadzic faces is one count of genocide over the massacre of Muslim men and boys at the UN-protected enclave of Srebrenica in July 1995, in which about 8,000 people died.
He also faces another genocide charge over the 44-month siege of Sarajevo.
Karadzic, who has repeatedly refused to enter pleas, has insisted that he is innocent.
His trial is expected to last for up to two years and he faces a maximum sentence of life in jail if convicted.
Harvey has previously served as defence counsel for two members of the Kosovo Liberation Army - Haradin Bala and Lahi Brahimaj, in two separate cases before the ICTY.
Bala was convicted to 13 years and Brahimaj to six years in prison for crimes in Kosovo in 1998.