He reassured Arab leaders, after a two-day summit, that the US was committed to protecting them in a time of "extraordinary changes".
Speaking at Camp David near Washington, Mr Obama said a nuclear deal with Iran was not a threat to Gulf nations.
A joint statement pledged new cooperation in many areas.
These included counterterrorism, maritime security, cybersecurity and ballistic missile defence.
The six-nation Gulf Cooperation Council (GCC) is made up of Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and UAE.
"I was very explicit ... that the United States will stand by our GCC partners against external attack,'' Mr Obama pledged at the end of the talks.
The president said he hoped that working together would help achieve "the kind of peace and good neighbourliness with Iran" that the nations present sought.
President Obama didn't give the Arabs the formal defence treaty some of them wanted.
But he went as far as he felt he could to reassure them that the US had their back, including a pledge of deeper military cooperation to counter what he called Iran's destabilising activities.
Significantly for Mr Obama, the Gulf nations said a nuclear deal with Iran would be in their security interests, if it was comprehensive and verifiable.
Such public support might make it a bit easier for the president to sell an Iranian agreement to a sceptical Congress.
So everyone got something of what they needed out of the summit, but fundamental differences remain over how they view Iran, and those will continue to play out.
So everyone got something of what they needed out of the summit, but fundamental differences remain over how they view Iran, and those will continue to play out.
The joint statement said that in the face of any aggression, the US would stand with the Arab nations "to determine urgently what action may be appropriate, using the means at our collective disposal, including the potential use of military force, for the defence of our GCC partners.
US relations with its partners in the Gulf have become strained due to the possible lifting of sanctions against Iran as part of a nuclear deal.
There was a reminder of these tensions as the summit concluded.
Saudi Arabia's foreign minister told reporters the Iranian navy firing at a Singapore-flagged tanker in the Gulf was a clear violation of international law.
But he went as far as he felt he could to reassure them that the US had their back, including a pledge of deeper military cooperation to counter what he called Iran's destabilising activities.
Significantly for Mr Obama, the Gulf nations said a nuclear deal with Iran would be in their security interests, if it was comprehensive and verifiable.
Such public support might make it a bit easier for the president to sell an Iranian agreement to a sceptical Congress.
So everyone got something of what they needed out of the summit, but fundamental differences remain over how they view Iran, and those will continue to play out.
So everyone got something of what they needed out of the summit, but fundamental differences remain over how they view Iran, and those will continue to play out.
The joint statement said that in the face of any aggression, the US would stand with the Arab nations "to determine urgently what action may be appropriate, using the means at our collective disposal, including the potential use of military force, for the defence of our GCC partners.
US relations with its partners in the Gulf have become strained due to the possible lifting of sanctions against Iran as part of a nuclear deal.
There was a reminder of these tensions as the summit concluded.
Saudi Arabia's foreign minister told reporters the Iranian navy firing at a Singapore-flagged tanker in the Gulf was a clear violation of international law.
