Marazion beach was cordoned off after a “possible unexploded ordnance” was discovered bobbing up and down in the surf on Sunday afternoon.
A beachgoer spotted the grey phosphorous flare and alerted the emergency services at about 3.15pm.
Highly trained Royal Navy Explosive Ordnance Disposal experts attended the scene along with members of Penzance Coastguard Rescue Team.
A cordon was put in place and the flare was “safely” detonated on Monday morning at about 8.50am.
A police spokesperson said: “Officers received a report of a possible unexploded ordnance which had washed up on Marazion beach, at around 3.15pm on Sunday.
“A cordon was put in place overnight to prevent access.
“The Coastguard and EOD teams attended this morning to safely detonate the item.”