John Hill, defendant James Fields' lawyer, made the remarks Monday in Charlottesville, the first day of jury selection.
Fields, 21, of Maumee, Ohio, is accused of having propelled his Dodge Challenger into a host of counter-partners at the Unite The Right rally of August 2017, having killed Heyer and injured several other people, police said.
Hill told prospective jurors that the court would hear evidence that Fields "thought he was acting in self-defense," WVIR reported.
CNN's attempts to contact Fields' lawyers to obtain their comments did not immediately succeed.
The selection of the jury should take two or three days and the trial will follow.
Fields is charged with first degree murder in Heyer's death. He is also facing five counts of maliciously injuring, three counts of serious injury and one count of failing to testify. Stop during an accident involving a death.
Clashes erupted in Charlottesville on the morning of the rally, forcing police to clear a park. The day was ruined by pepper spray, shouting and fist fights. Before the rally could begin, the police decided the protest was an illegal gathering and Governor Terry McAuliffe declared an emergency.
Fighting continued to erupt around the city. That afternoon, the authorities said that Fields had thrown his car into the crowd.
Fields pleaded not guilty to federal charges. It is difficult to know when he pleaded guilty, but a lawsuit would probably not be necessary if he had pleaded guilty.
Fields is held without bail in the Albemarle / Charlottesville Regional Prison.
Eliott C. McLaughlin of CNN contributed to this report.