Mara Buxbaum said the 37-year-old was taken to a west London hospital "as a precaution" and had been released.
"She's fine now," Buxbaum said. She declined to go into details of Ryder's illness.
British Airways said a flight from Los Angeles to Heathrow requested priority landing after a female passenger "required medical attention" and was taken to a local hospital for treatment.
"Our cabin crew looked after the female passenger as much as they could onboard and our passenger services staff also accompanied her to the local hospital," the statement said. "We wish her well."
Ryder, who began her film career as a teenager in 1986, earned back-to-back Oscar nominations in the 1990s for "Little Women" and the "The Age of Innocence." She's also known for youthful roles in films including "Heathers," "Edward Scissorhands" and "Girl, Interrupted. She has mostly stayed out of the headlines after her 2002 conviction for stealing several thousand dollars worth of merchandise from a Saks Fifth Avenue store in Beverly Hills.
Ryder had prescription drugs in her possession when she was arrested - but a drug charge was dropped after a doctor said he had prescribed the medication. A probation report stemming from the shoplifting incident showed that Ryder had dozens of prescriptions filled by 20 doctors over three years.