The Wye Valley Trust earned hundreds of thousands of pounds from parking fees last year, new figures show.
Hospitals across England received over £190 million from charging visitors and staff to use their car parks.
The GMB union called on the Government to enforce a blanket ban on NHS staff car parking charges.
Latest NHS England figures show Wye Valley NHS Trust earned £831,000 from parking charges in 2023 – up from £416,000 the year before.
This was made up of a combination of £439,000 from visitors, and £392,000 from staff.
The figures represent the gross income earned by the NHS and do not consider its own costs for providing car parking.
Nationally, there were over 446,000 parking spaces available at NHS facilities, a decline of 3% on last year.
Of these, 789 were available across all sites at the Wye Valley Trust, fewer than in 2022 – when there were 843.
Rachel Harrison, GMB national officer, said NHS workers have been "hammered by pay cuts, the pandemic and chronic understaffing", and called on ministers to enforce a blanket ban on NHS staff parking charges.
She added: "Of course trusts are short of cash after years of underfunding - but the money they claw back from charging staff to park is a drop in the ocean, while it can mean the difference between getting by and going under for hard-pressed staff."
The Liberal Democrats have called the fees a "tax on caring", and said the Conservatives were failing to deliver on a manifesto promise to crack down on unfair hospital car parking fees.
The party also warned the Government’s failure to properly fund local health services could lead to hospitals hiking parking charges on patients and staff.
Lib Dem health and social care spokesperson Daisy Cooper MP said: "It is unthinkable that Rishi Sunak is slashing NHS funding when hospitals are already on the brink.
"This will just make the cash crisis facing local health services even worse, forcing them to make more impossible choices in the years ahead."
A Department of Health and Social Care spokesperson said parking charges are needed to help manage capacity at NHS sites, and the fees charged must not be "significantly more than other hospitals in the local area".
"We will always support hardworking NHS staff and we have delivered on the commitment to provide free hospital car parking in England for those most in need", they said.
"Since 2022, all trusts have made hospital car parking free for blue badge holders, parents of very sick children, frequent attenders and NHS staff on night shifts, and this the first time that NHS hospital car parking has been free nationally across England for these groups."