After a 30-minute recess to reach a decision, chief judge Wenche Elizabeth Arntzen said lay judge Thomas Indrebø was unfit to continue because of comments he posted on a website the day after the attacks.
He is being replaced by one of two substitute judges, Anne Elisabeth Wisløff, already in court.
"The death penalty is the only fair outcome in this case!!!!" Indrebø wrote on a website the day after Anders Behring Breivik's July 22 twin attacks.
"Lay judge Indrebø used the Internet to say that (Breivik) should be sentenced to death," Arntzen noted, adding that this had "weakened confidence" in the court.
The case is being heard by two professional judges and three lay judges.
The death penalty does not exist in Norway.
Breivik risks either a 21-year jail term, which could then be extended indefinitely if he is still considered a threat to society, or closed psychiatric care, possibly for life.
The court immediately resumed proceedings with Breivik taking the stand to testify.