The 29-year-old pitcher is expected to stay in jail for a year while officials investigate a fatal shooting on New Year's Eve.
Simon's lawyer has said he was firing celebratory shots in the air. A bullet killed 25-year-old Michel Castillo, and injured his brother, 17-year-old Starlling Castillo, in the city of Luperon in Puerto Plata.
Defense attorney Dinosa Dilone said she plans to appeal the ruling.
"We think the judge acted in such a way to avoid a reaction from the media," Dilone said. "There is already forensic evidence that Simon's weapon did not cause the death of the victim."
A judge had earlier ruled that Simon could be held for up to a year pending trial. Simon could face up to two years in prison if convicted of involuntary manslaughter.
The Caribbean country's legal system allows for preventive detention. Human rights groups have long said the provision is abused and thousands of people are held unjustly for extended periods.
Simon joined Baltimore in 2008 but was quickly sidelined with an injury. He went 4-2 with a 4.93 ERA last season and had 17 saves.