Greenwood was arrested at an Evergreen hospital on Tuesday and charged with aggravated criminal sexual assault on Wednesday.
Police say a social networking site helped lead to his arrest.
Not only is William Greenwood accused of betraying his badge but doing so by preying on a young family member.
The 43-year-old Chicago police officer worked out of the Wentworth District on the city's South Side.
Prosecutors say Officer Greenwood repeatedly engaged in escalating "inappropriate sexual conduct" with a female relative who was 15 when the conduct is said to have begun. It allegedly happened over the course of four months earlier this year when Greenwood and the girl were home alone.
"There were numerous incidents of sexual penetration and sexual conduct that occurred within the city," said Katherine Muldoon, assistant Cook County state's attorney.
Prosecutors say the victim ultimately turned to technology for help. She logged on to Facebook and emailed a friend through the site, telling her of the alleged abuse. That friend pleaded with the girl to tell police.
On Tuesday, the victim sent a text message to her mother to report the abuse.
"After the first act of sexual penetration, she told her friend, and her friend just kept encouraging that you have to tell, you have to tell and get help and support," said Muldoon.
William Greenwood has been on the force for nearly nine years although he recently suffered an injury that had him off the street.
He's now been relieved of his police powers, and will spend the night in jail unless he can come up with $35,000, which is 10 percent of his bond.