Police were called to the London landmark around 2.20pm to reports that a man had brought in “suspected ordnance”.
Ordnance refers to military supplies including weapons, ammunition, combat vehicles, and maintenance tools and equipment.
The museum was evacuated while the police assessed the items – the exact identity of which is not known.
The museum then reopened before closing at 6pm as usual.
A Met Police spokesperson said: “Police were called at about 2.20pm on Monday, March 10, to Lambeth Road, Kennington, following reports that a man had attended with a number of items of militaria, including suspected ordnance.
“Emergency services attended and assessed the items. The incident has now been stood down.”
An Imperial War Museum spokesperson said: “At 2.45pm on Monday, IWM London was evacuated at the request of the Metropolitan Police after a member of the public brought a Second World War era object into the museum.
“IWM London was temporarily closed while the police were on site. The museum has now reopened to visitors.”
