It happened between 3am and 3.30am on Saturday on North Bridge in Edinburgh.
Two men were seen walking in the area holding the extinguisher before it was found shortly afterwards in Market Street.
No one was injured but Lothian and Borders Police said there could have been fatal consequences if someone was hit and are trying to to trace the two men.
The first is white, 5ft 5in, skinny and has medium-length, dark hair. He was wearing a dark red or maroon jumper and dark jeans. The other man is also white, 6ft and skinny with short brown hair. He was wearing a light jumper and dark jeans.
Sergeant Carol McKenzie said: "This was an incredibly reckless act of stupidity which could have had potentially fatal consequences had anyone been struck by the fire extinguisher as it fell from North Bridge to Market Street.
"Despite the time that this incident occurred, there may have been a number of witnesses returning home who can help us identify those responsible.
"Anyone who was in or around North Bridge or Market Street in the early hours of Saturday morning and remembers seeing anything suspicious is asked to contact police immediately."