Gavin Williamson told MPs regulars and reservists will be available to support any Government department with contingency plans.
His remarks came in the Commons after Tory MP Will Quince (Colchester) asked if there had been approaches from other Government departments about using the UK's "world-class armed forces personnel in the event of a no-deal Brexit".
Mr Williamson replied: "We've as yet not had any formal request from any Government department but what we are doing is putting contingency plans in place, and what we will do is have 3,500 service personnel held at readiness - including regulars and reserves - in order to support any Government department on any contingencies they may need."
Labour MP Ian Murray (Edinburgh South), a supporter of the pro-EU Best for Britain campaign, said in a statement outside the Commons: "This is the reality of a no-deal Brexit: soldiers on the streets, medicines being stockpiled in the NHS, and airports and ferry terminals grinding to a halt.
"This is scary tactics, pure and simple."
Liberal Democrat MP Jamie Stone, the party's armed forces spokesman, said in a statement: "The Tory Government wholeheartedly accept that a no-deal Brexit will create chaos and mayhem.
"This announcement is simply an attempt to scaremonger MPs, businesses, and the British public into supporting the Prime Minister's deal.
"The Government must stop playing games with our future and diverting public resources to solve an outcome they are able to prevent."
He added a second EU referendum was the "only real way out" of the Brexit deadlock.