Dressed in black and wearing masks over their faces, the two reached the top of the building's antenna, 443 meters above the ground in downtown Manhattan.
The banner, with the text "when the power of love overcomes the love of power, the world can reach peace," waved in the wind as they climbed back down at noon on Wednesday, local time.
In pictures that the couple themselves posted on social media, the man then proposes to the woman as they stand on a platform below the antenna.
It is unclear how they managed to reach the top of the skyscraper, which rises significantly higher than the nearest public area of the 102-story building.
They were both later arrested.
