Francisco Palomo was shot at least 12 times by gunmen in Guatemala City.
Mr Palomo, 63, defended Gen Rios Montt against charges connected to the killing of more than 1,700 indigenous Guatemalans in the 1980s.
The general was sentenced to 80 years in prison two years ago but his conviction was overturned.
A retrial scheduled for January has been postponed indefinitely because of a legal dispute.
Mr Palomo was shot in the chest and face while driving his car at about midday on Wednesday, emergency services spokesman Raul Hernandez said.
Witnesses said the gunmen were on a motorcycle.
"The victim, still alive, tried to flee the attack and drove a few metres before crashing into a tree," Mr Hernandez said.
The motive for the killing is still not clear.
A lawyer who represented former Guatemalan military leader Efrain Rios Montt against genocide charges has been shot dead.