"This milestone in human spaceflight began the journey on the road to human exploration of the universe," the spokesperson said.
On April 12, 1961, Gagarin became the first person to travel into space aboard the Vostok spacecraft. Cosmonautics Day is celebrated in Russia and some other post-Soviet republics on April 12 to commemorate Gagarin's achievement.
The NASA spokesperson highlighted that many significant accomplishments for the Russian and US space programs have occurred over the decades as shown through the international partnership on the International Space Station, and will continue in the future to inspire new generations of explorers.
Earlier on Friday, Russian cosmonauts Oleg Kononenko, Nikolai Chub and Alexander Grebenkin, who are currently aboard the International Space Station, congratulated Russians on Cosmonautics Day.