Part of the arboretum was closed on Thursday as the bomb squad carried out a controlled explosion.
The shell was found in a garden in Lichfield Street at about 1.45pm and the detonation was at 6.45pm.
Ops Support Unit WMP tweeted on Thursday saying: "After a WW2 shell was discovered earlier today U4B assisted Walsall police with disposing of the item with the expertise of 11 Explosive Ordnance Disposal and Search Regiment. Thanks to our colleagues at the National Police Air Service and West Midlands Police Dogs for their help also."
Walsall Council tweeted at 6.20pm on Thursday saying the arboretum extension would be closed for the rest of the day.
A spokeswoman for West Midlands Police said: "We were called after a weapon shell was found in a garden in Lichfield Street, Walsall, around 1.45pm yesterday.
"Experts from the bomb disposal unit were called in and a controlled explosion was carried out at a disused golf course at Walsall Arboretum by 6.45pm."