Royal United Hospitals Bath made millions of pounds in profit from charging patients and staff to park in the year to March, new figures show.
The GMB union, which represents many NHS staff, said it was "perverse and grotesque" to force them to pay to park at work.
New figures released by NHS England show Royal United Hospitals Bath NHS Foundation Trust received around £2.6 million from parking charges across its various sites.
This was made up of £2.4 million from charging patients and visitors, and £214,000 from staff.
Across England, trusts received £271 million from parking charges, a 12% increase on 2023-24.
At Royal United Hospitals Bath income from parking increased by 62% from £1.6 million a year earlier.
Brian Morton, associate director for employment relations at the Royal College of Nursing, said: "For nursing staff, the cost of parking takes too much of their pay.
"Staff are having to pay to park at work and can even be hit with fines when they stay late caring for their patients. This simply cannot be right.
"Staff work around the clock to be there for their patients – and working odd shift times, means using public transport is not always possible.
"Trust leaders must do more to provide free parking for staff and not leave them out of pocket just for coming to work."
The data also shows the cost of administering NHS car parks rose by 9%, a slower rate than the income they generated.
Nationally, the NHS spent £84 million administering parking, giving it a total net income of £187 million.
Managing parking cost Royal United Hospitals Bath a total of £270,000, meaning overall it gave it a net income of £2.4 million.
Sharon Wilde, national officer at the GMB said: "When the charges have risen faster that the cost of maintaining the car parks, it looks like the worst kind of profiteering.
"Health workers deal with punishing workloads and chronic understaffing – they need help and support.
"Paying to park while performing vital, lifesaving work feels like the ultimate kick in the teeth."
A Department of Health and Social Care spokesperson said local trusts are responsible for car parking space numbers, rates for parking and charging methods, but all parking charges should be "reasonable for the local area."
They added: "Free parking is available for those in greatest need, including all NHS staff who work overnight.
"While hospital car park charges are the responsibility of individual NHS trusts, we expect all charges to be reasonable and in line with the local area".