Volunteers for the Water of Leith Conservation Trust were wading in the water at Warriston last week, looking for invasive species, when they discovered the Glock semi-automatic pistol.
The weapon appeared to have been in the river for some time.
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The volunteers handed the gun over to the armed police unit at a local police station.
A Police Scotland spokesperson said: “Officers were called to a report of a firearm found in the Water of Leith in the Warriston area of Edinburgh at 1.20pm on Thursday 21 July, 2022.
“The firearm has been made safe and further enquiries are ongoing.”
On Twitter, the Water of Leith Conservation Trust wrote: “”A day of contrasts down at the Rocheid path today for our staff.
“From a peaceful wade for invasive species whilst being watched by the chess men of the path.
“Then a visit to Gayfield Police station to drop off a recovered fire arm from the river.
“Never the same day twice.”
This is not the first time a weapon has been pulled out of a body of water in Edinburgh. Earlier this year, Rope Walk was closed off by bomb disposal officers after magnet fishermen pulled out a handgun and a suspected explosive device from the canal.
Glock pistols are used by military and law enforcement around the world, but are also popular amongst criminals.
They have no safety catch and are the most reliable semi-automatic 9mm pistol in production.