Members of her family flew in to Cape Cod Hospital to be with the 88-year-old, including California Gov. Arnold Schwarzenegger, the husband of her daughter, Maria Shriver, a source told CNN.
No further information on her condition was immediately available.
The sister of President Kennedy, Bobby Kennedy and Sen. Ted Kennedy, Shriver is renowned in her own right as the founder and honorary chairwoman of the Special Olympics, which helps 3 million people with intellectual disabilities in nearly 200 countries around the world.
"The Special Olympics movement is saddened to hear that our founder, Eunice Kennedy Shriver, is hospitalized. Our thoughts and prayers are with Mrs. Shriver and her family at this time," organization President Brady Lum said in a statement Friday.
"Every day, our Special Olympics family continues to work tirelessly around the world to bring her powerful vision to life to change the lives of people with intellectual disabilities, using sports as the catalyst for respect, acceptance and inclusion," he said.
The Special Olympics grew from Camp Shriver, a summer day camp begun in 1962 in her backyard, where people with intellectual disabilities could explore their capabilities in a variety of sports and physical activities, according to the organization's Web site.