Sussex Police were called to Lansdowne Road at 9.35am after reports of a 'WWII-like' object .
Royal Navy Bomb Disposal was called to the scene to assess the item but found that it was not an explosive device at all.
The A259, which had been closed as a precaution, is now open again and residents who had been evacuated have been allowed to return home.
Arun Police tweeted: "The un-exploded WWII device in Littlehampton has now been assessed by the military and deemed safe to the public. All roads will be open shortly. Thank you for your patience. #ce204 #Esection #teamworkmakesthedreamwork"