Philip Markoff, 24, was found unresponsive in his cell Sunday morning in the Nashua Street Jail, the Suffolk County district attorney's office said in an e-mailed statement, and he was pronounced dead at about 10:15 a.m.
"Markoff was alone in his cell, and all evidence collected thus far indicates that he took his own life," the statement said.
Authorities will investigate to determine the facts and circumstances surrounding his death, the district attorney said.
Saturday would have been Markoff's first wedding anniversary, but his nuptials were canceled after his arrest.
Markoff, a former Boston University student, pleaded not guilty in the fatal shooting of Julissa Brisman, of New York City, and the armed robbery of a Las Vegas woman. Both crimes happened at Boston hotels within the span of four days in April 2009. Rhode Island prosecutors also accused him of attacking a stripper that week.
His trial in the Massachusetts cases was expected in March.
Markoff's lawyer, John Salsberg, said he was shocked and saddened about his client's death. He would not comment further.
Markoff had met the women through advertisements for erotic services posted on Craigslist, a classified advertising Web site, prosecutors said.
A lawyer said Brisman's family was "shocked and dismayed" to hear of Markoff's death.
"Their grief for Jullissa is as fresh today as the day over a year ago when Markoff took Julissa away from them," Boston attorney Djuna Perkins said in an e-mail. "The long-awaited criminal prosecution was their only opportunity to confront him, and now he has taken that away as well."
Still, she said, the family will pursue other "avenues to seek justice" and to help ensure others do not fall victim to violence. She said the family has asked a federal prosecutor in New Hampshire to investigate the gun shop used in the crime.
Markoff was engaged at the time of his arrest. His fiancee, Megan McAllister, ended the relationship with Markoff after visiting him in jail, and their wedding, scheduled for Aug. 14, 2009, was canceled.