Donald Trump's inauguration ceremony is relying on a private security company ... and Uncle Sam is paying a pretty penny to keep the incoming President safe.

TMZ obtained federal procurement docs that show the Secret Service has hired a company called Arcus Group LLC to help out with Monday's inauguration. The company scored an $8.1 million contract for the job ... and the firm is tasked with securing the perimeter.

Sounds like taxpayer money is being put to good use here ... Arcus Group boasts they are specialists in temporary security perimeters for high profile situations ... and Trump's inauguration fits the bill.

The firm says they provide top-notch stuff like anti-scale fencing, barricades, golf carts and gators, mobile commands posts, light towers, all types of barriers, tents, mobile power generators and perhaps most useful of all ... portable bathrooms!!!

Trump had a few attempts on his life during the campaign so the Secret Service needs all the help it can get here.

Costs are going up ... the Secret Service spent $7.5 million on private security for Joe Biden's inauguration ceremony. Inflation's a bitch!!!