Figures from 170 organisations across England - including hospitals and mental health trusts - show some spent more than £200,000 a year while others spent just a few hundred pounds.
The cost is for transferring patients by private taxi when they are too ill, frail or unable to travel to and from appointments by themselves.
A taxi may be paid for by the NHS if there is a shortage of patient transport services at the hospital, including ambulances or car drivers.
Some NHS trusts also have contracts with private taxi firms.
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The figures come from data supplied to the Health and Social Care Information Centre (HSCIC) for 2014/15 and from individual trusts.
Among the high spenders are the North East Ambulance Service NHS Foundation Trust, which spents £722,000 on taxis as part of its service to NHS hospitals.
Elsewhere in the North East, South Tees Hospitals NHS Foundation Trust spent £617,008.
Katherine Murphy, from the Patients' Association, said: "There is an exorbitant amount of waste and inefficiency in the NHS and we've got to stop it.
"Surely, by putting some time aside for planning patients' needs, we could use hospital transport far more effectively and not continue to waste money on private taxis?"
A spokeswoman for the North East Ambulance Service said: "The vast majority of taxi services are procured as part of non-emergency patient transport services to cover pre-arranged non-urgent journeys; for example, transport to and from hospital appointments.
"Where clinically safe to do so, we also use taxi services for patients who are triaged as needing to go to hospital, but who do not need medical intervention on the way there."
She added: "We have purposefully gone out to contract with reputable taxi services who work to specification so that we know they are safe and caring."
A spokeswoman for South Tees Hospitals NHS Foundation Trust said: "A large proportion of our taxi costs are incurred by patients receiving treatment from our specialist renal dialysis service, which includes patients in Darlington and North Tees, as well as the South Tees area.
"The spend in specialist services alone was£465,000 due to renal services."
Unison union general secretary Dave Prentis said: "While taking ill and elderly people to and from hospital in a cab is clearly preferable to making them take the bus or train, taxis are no substitute for an ambulance or a hospital car containing someone who is medically trained.
"If something were to go wrong, it could be dangerous for the patient, and wouldn't be very fair on the cabbie either."
Rehana Azam, GMB acting national secretary for public services, said: "The Government keep saying NHS funding is protected, yet sick people are being put in taxis because there are not enough NHS vehicles to safely transport them."
A spokesman for NHS England said: "For several decades it has been common to contract with non-emergency patient transport providers so as not to tie up emergency ambulances on routine but necessary trips to hospital by patients who qualify for public support."