"I have no illusions about how hard it will be to achieve this objective, but for the sake of our national security and the peace and security of the world, now is the time to give diplomacy a chance to succeed," he said.
Under the agreement, which was reached in November, Iran will eliminate stockpiles of enriched uranium and dismantle equipment used in the enrichment.
Iran will also not install additional centrifuges, and inspections of the nuclear facilities will ensure that Iran is complying with the agreement.
In response, the United States and its partners will lessen the sanctions on Iran.
"Meanwhile, we will continue to vigorously enforce the broader sanctions regime, and if Iran fails to meet its commitments we will move to increase our sanctions," Obama said.
Seyed Abbas Araqchi, Iran's deputy foreign minister for legal and international affairs, and European Union Chief Helga Schmid concluded four rounds of talks on Geneva on Friday ahead of the agreement, the Tasnim news agency reported.