0450 GMT February 18, 2018
Johnson made the announcement on Friday, citing increasingly sophisticated foreign-backed cyber attacks and an election infrastructure that is "vital to our national interests."
"Given the vital role elections play in this country, it is clear that certain systems and assets of election infrastructure meet the definition of critical infrastructure, in fact and in law," Johnson said in a statement.
"Particularly in these times, this designation is simply the right and obvious thing to do," he added.
Johnson said election infrastructure included polling places, vote tabulation locations and storage facilities, in addition to the technology involved in the process, including voter registration databases, voting machines and other systems.
The designation places responsibilities on the Department of Homeland Security to identify sectors as critical infrastructure and prioritize them, including energy, financial services, health care, transportation, food and agriculture and communications.
However, the designation does not require entities that are determined "critical infrastructure" to participate. Most of the US critical infrastructure is owned by the private sector.
Such a change does not require presidential authorization, and only requires the secretary to first consult with the assistant to the president for homeland security and counterterrorism.
Johnson announced the designation on the same day a declassified US intelligence report said Russian President Vladimir Putin "ordered" a campaign aimed at influencing the outcome of the 2016 US presidential election.
The report, released by the Central Intelligence Agency (CIA), the Federal Bureau of Investigation (FBI) and the National Security Agency (NSA), said Putin personally ordered his government to help incoming US President Donald Trump win the presidential election.
The report claims Russia "sought to help" Trump by running a smear campaign against Hillary Clinton, his Democratic rival.
Emails allegedly stolen by Russian hackers from the Democratic National Committee (DNC) and Clinton campaign chairman John Podesta were leaked to the media by WikiLeaks prior to the election, embarrassing the Clinton campaign.
Last week, outgoing President Barack Obama ordered a series of economic sanctions against Russia, as well as expulsion of 35 Russian diplomats over the hacking allegations.