The Polish president made the statement on Wednesday at a Nato summit in Madrid. Earlier in the day, Turkey, which initially opposed Sweden and Finland joining Nato, agreed to their membership.
"We have managed to maintain unity," Duda said. "The North Atlantic Alliance is united and is driven by solidarity.
"The green light for Sweden and Finland to join Nato is certainly a historic moment from the point of view of the whole Alliance," he added.
Duda said that Nato's invitation of the two Nordic countries was also very important for Poland.
"For us, for Poland, for our part of Europe and for the eastern flank of Nato - it is an absolutely fundamental decision," he said.
"Two very powerful states, with great histories, but also ones that possess very serious military capacities, Sweden and Finland, countries of the Baltic Sea basin, will be accepted into the North Atlantic Alliance, which basically means that the border of Nato's eastern flank, the border of the flank with Russia, will be extended by over 1,600 kilometres," Duda said.
According to the Polish president, "today, if Russia decided to attack Nato's eastern flank, the danger to Russia from Nato's defensive response is growing immensely, because more strong armies are joining the Alliance.
"For us, for Poles, for Poland's defence and security, it is very important news," he added. (PAP)