The blunder was caught live on camera outside Downing Street this morning (September 1), as he arrived for a cabinet meeting with the Prime Minister.
The 50-year-old had just arrived at the Foreign and Commonwealth Office, where strict social distancing measures are in force, when he was spotted eagerly shaking hands with an associate outside.
The gaffe happened just hours before the Government is expected to make an announcement urging the British workforce to begin returning to workplaces.
The Government has repeatedly advised against shaking hands to help prevent coronavirus from spreading.
Ben Wallace has been the Conservative MP for Wyre and Preston North since he was elected in May 2010. He had formerly been MP for Lancaster and Wyre in 2005.
As secretary of state for defence, Mr Wallace has responsibility for the UK's strategic operations and defence planning, including international partnerships with the US, France, Germany, Saudi Arabia and the Gulf, as well as NATO's National Security Council.
Mr Wallace has been approached for comment.