Donald Trump Inauguration's Gonna Be Hot, But It's Too Cold Out ... Moving The Party Indoors!!!

Donald Trump's looking to keep his supporters safe on Inauguration Day ... moving Monday's events indoors 'cause it's freezing in the nation's capital -- after the country dropped a bag on security procedures for the event.

The president-elect -- who will officially become the 47th POTUS on Monday -- jumped on Truth Social to announce the change ... writing that his job is to protect all Americans, including during his inauguration.

Trump explains the windchill in D.C. combined with the already frigid temperatures could lead to record lows ... and, it wouldn't be safe for people to stand around for hours, waiting for his big moment.

That said, DJT knows some people might still be determined to just be in D.C. the day he rises to the highest office in the land for a second time ... and, he's recommending warm clothes.

Trump says the plan is to open up Capital One Arena -- home of the NBA's Washington Wizards and NHL's Washington Capitals -- for people to watch a live stream of the event ... and, 47 says he will join them later in the day.

Trump notes Ronald Reagan's 1985 inauguration also took place inside the Capitol Rotunda, just like his own will ... also due to bad weather.

While moving the ceremony is almost certainly the right call, it's going to cost a pretty penny ... 'cause a pricey security company was hired for the event.

We broke the story ... the Arcus Group LLC received a little more than $8 million from the Secret Service to protect Trump during the event -- but, most of what they were providing included measures like barricades, anti-scale fencing and portable bathrooms -- none of which are necessary inside the Capitol.

Hopefully, the Secret Service held on to the receipt.