Police said the cause of death hasn't been determined.
The Maricopa County Sheriff's Office said deputies found the body of 39-year-old Gabrielle Dawn Schoeneweis on the floor of the master bedroom in the family's suburban home shortly after noon.
Authorities said her 14-year-old daughter called the sheriff's office to report that she had found her mother lying there and unresponsive.
A team spokesman said Schoeneweis had left the team, which was playing a doubleheader in Florida on Wednesday night, and was returning to Phoenix.
"At this point, anything we can do to support Scott and his family, will be our focus," said Diamondbacks general manager Josh Byrnes, who was with the team. "I have talked to Scott Boras, the agent, a few times. We're trying to get the facts the best we can. It's devastating news. Everyone is taking it very hard."
The 35-year-old Schoeneweis came to the Diamondbacks last December in a deal with the New York Mets, who agreed to pay $1.6 million of his $3.6 million salary for 2009.
Schoeneweis, a survivor of testicular cancer, is 45-55 with a 4.89 ERA in 11 seasons with five teams, the last two in New York.
Scott and Gabrielle Dawn Schoeneweis celebrated their 10th anniversary in January. They had three children together, and she already had a daughter when they were married.
"His world was just rocked. He and his kids, his family, his extended family, his in-laws, everybody's lives are changed forever. He took it as expected, very hard," Diamondbacks manager A.J. Hinch said after Arizona lost 8-6 to Florida in the first game. "As I told the team, just give him as much support as you can. Really, tell people that you love them if you do. You can never expect something like this to happen."
Arizona starter Doug Davis said he was told about 30 minutes before making his start against Florida.
"A.J. brought everybody together and told us about it. Obviously our thoughts and prayers are with him," Davis said. It is a tough time for not just him, but all of us and his family. It's tragic. You don't know what else to do, but to pray for him."
Francisco Rodriguez, who teamed with Schoeneweis in the Angels bullpen in 2002 and 2003, said he was "speechless" when he heard the news.
"I'm sad for him," Rodriguez said. "I can't imagine what he's going through right now, all the pain he's going through."
Former Mets teammate Ryan Church added: "He's always been able to battle through adversity because he's had cancer before. Everybody needs to be thinking about him and his family and just pray, pray a lot."
Mets manager Jerry Manuel said he still considers Schoeneweis part of his family. "I mentioned that to our people, that we've got to reach out and do all we can to help support him at this time," Manuel said. "And whatever he needs from us, we'll be here for him."