Called 'Baby Doe" by  Boston police, the four-year-old girl was found by a woman walking her dog on 25 June.
The National Center for Missing and Exploited Children created the image in the hope that someone who knew her will come forward.
She was found with a zebra-print blanket and wore polka-dot leggings.
The centre posted an updated version of the image to add a pair of earrings. She is believed to be either white or Hispanic, weighed about 30lbs (13.6kg) and stood 3.5ft (1m) tall.
Police believe she died not long before her body was discovered inside a large plastic bag on Deer Island in Boston harbour.
A post-mortem examination hasn't yet determined the cause of death, according to the Boston Globe newspaper.
Officials said the case has received a massive response on social media and drawn thousands of new visitors to government websites.
"Every now and then we have a case like this, and that probably has a lot to do with the age of the child, the circumstances in which the child was found," Suffolk District Attorney Daniel Conley told the Washington Post.
"Sometimes it touches the hearts of many and it motivates the public to get motivated to help."
A computer-generated image of a girl who was found dead has been shared millions of times online as US police search for clues about how she died.