The Staffordshire Regiment Association said it paid Â£64,000 to buy a 10-year lease on The King's Head in Lichfield.
The regiment was founded in 1705 when Col Luke Lillingston raised a force that later became the Staffords.
Capt Dave Donnelly said: "It's brilliant. We believe we have bought a piece of history."
He said the purchase was prompted by a "mad idea" by ex-warrant officer Derek Watts, from Wolverhampton, who went on social media to drum up support.
Within a fortnight, he had managed to raise enough pledges to buy the pub.
"We have reunions in the pub annually, so we didn't want it becoming a bed and breakfast or anything like that," said Capt Donnelly.
"The support to get this off the ground has been immense. We don't know how many associations run pubs, but what is unique is that we were actually founded there."
The pub will reopen - following a refurbishment - on March 25, 312 years to the day from when the regiment was formed.
It will serve two ales created in honour of the regiment - one named Col Lillingston and the other named Watchman, after the association's mascot - and will be managed by a former regiment man.
It will be opened by Lichfield's Lord Mayor and a descendent of Col Lillingston. The association will march from Lichfield Cathedral to the pub and will play the Last Post.

Jamie Hall, 22, made no plea or declaration during a private hearing at the city's sheriff court on Monday.
Prosecutors allege he carried out the sexual assault in the city's Spring Gardens.
Mr Hall, from Edinburgh, was remanded in custody ahead of his next court appearance.

13 October 2016 Last updated at 00:23 BST
Many analysts think Sony's kit will outsell rival headsets from HTC and Facebook's Oculus division because of its lower price and the fact that more than 43 million PS4 consoles have been shipped.
Even so, they add, it is likely to remain a niche product for some time to come.
Strategy Analytics forecasts that only 3% of Brits will own a VR headset this year and the PlayStation model will account for just 7% of that number. The bulk, it says, will instead own more basic "shells" into which they can slot a smartphone.
BBC Click's Marc Cieslak put the PS VR to the test and had reservations of his own.
More at BBC.com/Click and @BBCClick.

Boxing on the undercard of Carl Frampton's loss to Leo Santa Cruz, the 26-year-old from Prestonpans went the distance for the first time.
The Commonwealth super-lightweight champion now has eight wins since turning professional.
Despite being deducted a point for low punches he won by 78-73 on two cards and 79-72 according to the third judge.
Taylor described the experience of fighting in Las Vegas as "brilliant", but felt he did not hit top gear in the ring.
"It was far from my best performance," the Scottish fighter said. "I was a little bit too eager to impress on the big stage.
"I felt a little bit tight and tense. I couldn't get my footwork going, couldn't get flowing. I never felt like I was in any danger, I felt very comfortable in the fight. I felt like I never got out of second gear.
"He was very awkward, very hard to nail clean. When I did catch him he was very good at spoiling me. It was a good learning fight for me and to good get the eight rounds in."
Veterans of a historical Army regiment have bought the lease on the pub where it was founded.
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A man has appeared in court charged with raping a 37-year-old woman in Edinburgh on Boxing Day.
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The PlayStation's virtual reality headset is making its way into gamers' homes - the last of this year's three big VR hardware releases.
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Josh Taylor remains unbeaten after a comfortable points victory over Alfonso Olvera on his Las Vegas debut.