U.S. District Judge George A. O’Toole Jr. did not describe the illness or say which of the seven women and five men was sick. But he said he expected the juror to be well for testimony to resume next week.
Defense lawyers are expected to wrap up their arguments by midweek. The jury then must decide whether to sentence Tsarnaev, who was found guilty of 30 charges in the April 2013 bombing case, to life in prison without parole or the death penalty.
Defense lawyers have argued that Dzhokhar, who was 19 at the time, was misled by his older brother, Tamerlan Tsarnaev, and thus should be spared the death sentence.
They portrayed the older brother, who was killed in a shootout with police after the bombing, as a domineering figure and Muslim extremist who planned the attack and built the two homemade bombs that exploded in the crowd at the race's finish line.
The government contends that Dzhokhar, who placed one of the bombs, has shown no sign of remorse for the blasts that killed three people and injured more than 260 others.
Alexa Guevara cried on the witness stand Wednesday as she described her friendship with Dzhokhar on the campus of the University of Massachusetts Dartmouth. When she left the courtroom, she broke into sobs that could be heard down the hallway.
Prosecutors plan to bring her back to the stand for cross-examination when the trial resumes.
For more on the trial, follow @RickSerranoLAT