'Big Drum' was a British one of several Allied naval deceptions for the 'Neptune' (iii) assault phase of 'Overlord' (5/6 June 1944).

'Big Drum' was similar to 'Glimmer' and 'Taxable', the other D-Day naval deceptions, but without any air component. The unit entrusted with this naval undertaking was Special Task Force C, which comprised just four harbour defence motor launches. The object of the effort was to operate as a distraction on the western flank of the invasion fleet and therefore sow an increased measure of confusion in German minds about the real width of the assault area targeted by 'Neptune' (iii). Attached to Force 'U', which was the westernmost convoy of the invasion fleet, the task force was originally to operate radar jamming equipment as it approached the French coast, holding 2 miles (3.2 km) off shore until first light. However, after the Germans failed to respond, the launches moved to within 1.5 miles (2.4 km) of the coast. No response, either in the air or on the shore, was observed, and the task force returned safely to Newhaven.

There was also a nuisance and diversionary air raid, not officially part of 'Big Drum', on Cherbourg on the northern coast of the Cotentin peninsula of German-occupied northern France, using aircraft to drop 'Window' chaff as they flew slowly to give the impression that large numbers of troop-carrying aircraft were approaching and therefore diverting the attentions of the German defence away from the points at which the real Allied airborne landings were to be made as part of 'Neptune' (iii).