The crime came as a surprise to the authority, who claim it will cost £1,000 to replace the door.
The Loch Lomond and The Trossachs National Park were notified by cleaning staff yesterday morning the toilet door at the public facilities in Inveruglas, Argyll and Bute, was missing.
A spokeswoman for the park authority said: “We were appalled to discover that someone had actually gone to the effort to steal the accessible toilet door at our site at Inveruglas.
“Not only does this mean we have to spend additional funds replacing the door that could be put to better use elsewhere, it also makes it more difficult for people with disabilities to use our facilities.
“We have already ordered a new door and hope to reopen the toilet as soon as possible, but it is likely to remain closed for at least two weeks.
“The male and female toilets are open at the site and their are accessible toilets at Tarbert and Crianlarich.”
The park authority confirmed the toilets at Inveruglas were locked overnight, cleaned several times throughout the day and often inspected by passing National Park rangers.
They shared the news on Facebook earlier today, stating: “Someone has stolen a toilet door from our visitor site at Inveruglas. And not just any toilet door, they’ve taken the accessible toilet door.
“We know some of our facilities might be in need of some upgrading, but this isn’t what we had in mind.
“So as well as making it more difficult for people with disabilities to use our facilities, we also now have to spend money on a new door that could be better spent elsewhere.”
They have appealed for anyone who was at Inveruglas earlier this week to let them know if they had seen anything.
The national park added: “A new door has been ordered and will be fitted ASAP.
“We apologise for any inconvenience in the meantime.”