A North Korean referred to only as "Kim" was paid to carry out an attack with biochemical substances, the ministry of state security said.
The plot was foiled, it said, but gave no details on the fate of "Kim". The CIA declined to comment and South Korea has issued no statement so far.
The North's claim comes amid continued high tension on the Korean peninsula.
US President Donald Trump has promised to "solve" North Korea and stop it developing nuclear weapons.