Sally Baker, of Meadow Road, Worthing, was walking her 14-month-old rescue dog, Newt, when she discovered the object on East Worthing beach.
Part of Brighton Road, between Ham Road and Brougham Road, has been closed as a precautionary measure.
She said: “I took the dog out before I go to work and she was trying to eat and dig it. It was about 50cm long and I didn’t want to touch it myself.
“There’s all sorts of things washed up and it just looked like another bit of rubbish, especially covered in mussel shells, so it was only when I got closer I thought ‘uh oh’, that’s not good.”
A spokesman for Sussex Police confirmed the bomb disposal unit was notified.
She said: “Police and coastguard were called to a report of ordnance washed up on the beach opposite the junction of Ophir Road, East Worthing, at 8.25am on Wednesday.
“The shell is 20 metres from the Brighton Road so a cordon has been set up and Brighton Road between Ham Road and Brougham Road has been closed.
“The bomb disposal unit has been notified and will be attending.”
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