The female soldier, believed to be an Army Reserve volunteer, was flown back to the UK by a specially equipped Royal Air Force plane and has been admitted at The Royal Free Hospital in north London.
The other two military workers have not been diagnosed with Ebola but have also been admitted to the hospital's isolation unit as a precaution, and can expect to be quarantined for 21 days.
Between 600 and 700 UK defence personnel are based in Sierra Leone as part of efforts to tackle the largest ever outbreak of Ebola.
Mark Francois, UK minister for the Armed Forces, said: "The wellbeing of our service personnel remains our overriding priority. This includes the individual directly affected and their four colleagues, for whom precautionary measures are now being taken.
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"We are very proud of what our servicemen and women are doing and our thoughts are with their five colleagues and their families at this time."
Neither has been diagnosed with Ebola.
Dr Jenny Harries, Ebola incident lead at Public Health England, said: "There is no risk to the general public and the risk to the UK continues to be very low. Our thoughts are with the healthcare workers and their families."
The Royal Free Hospital said: "We are working with Public Health England to provide clinical assessment of a military healthcare worker who has tested positive for the Ebola virus.