Ben Wallace said the prospect of the US stepping back from its international leadership role under Donald Trump “keeps me awake at night”.
It may force the UK to rethink its assumptions about defence, he added.
His comments come as the UK prepares to carry out the “deepest review” of Britain’s security, defence and foreign policy since the end of the Cold War.
“I worry if the United States withdraws from its leadership around the world,” Mr Wallace told the Sunday Times. “That would be bad for the world and bad for us. We plan for the worst and hope for the best.”
He said the defence review should be used to make the UK less dependent on the US in future conflicts.
“Over the last year we’ve had the US pull out from Syria, the statement by Donald Trump on Iraq where he said Nato should take over and do more in the Middle East,” Mr Wallace said.
“The assumptions of 2010 that we were always going to be part of a US coalition is really just not where we are going to be.
“We are very dependent on American air cover and American intelligence, surveillance and reconnaissance assets. We need to diversify our assets.”
Mr Wallace said last month there was a shortfall of funding in the Ministry of Defence’s budget.
The MoD was given £2.2bn, a rise of 2.6%, in September’s spending review.