Blinken's reassurance came hours after Pyongyang fired multiple short-range ballistic missiles towards South Korea's eastern waters.
The US diplomat met President Yoon on the sidelines of third Summit for Democracy being held in South Korea's capital city.
Summit for Democracy is President Joe Biden's initiative and is being hosted by Seoul this year from March 18-20. It brings together government officials, NGOs and civil society members.
Co-hosted by South Korea, Britain, the United States and Ecuador, the conference brought together top government delegations of some 30 countries.
Blinken previously called for continuous close coordinantion and communication between US and South Korea to strengthen their "extended deterrence".
Meanwhile, President Yoon welcoming Blinken said recent senior-level exchanges shows the strength of their alliance that has evolved into "global comprehensive strategic alliance".
South Korea and US had last Thursday wrapped up 10 days of annual joint military drills which has been long condemned by Pyongyang and dubbed as rehearsals for takeover.
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