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.
Veterans of a historical Army regiment have bought the lease on the pub where it was founded.