Keepers discovered Muchana, an 8-year-old male, in his sleeping quarters. The keepers, joined by members of the veterinary staff, tried to revive him without success.
Bill Houston, an assistant general curator, told the St. Louis Post-Dispatch the rope was braided, and Muchana appeared to have separated the strands.
Most visitors Saturday were not aware of the death, Houston said, because the Great Apes area has been closed to the public for renovation.
The death was the second among primates in the zoo in two weeks. The cause of death of Cinders, a hairless chimpanzee, had not been determined pending the results of tests.
Houston said Jade, an Asian elephant, appeared to be recovering from a herpes infection.