"The two movements have agreed to join and coordinate all political, military and social forces, their international relations and to double their combat capacity in a collective body under the name, the Alliance of Revolutionary Forces of West Sudan," they said in a press statement on Friday.
"This union will strengthen the solidarity, cohesion and unity of the people of Sudan in general and that of the west in particular," the document said.
"It will further strengthen the position of the armed movements in (peace) negotiations" currently under way in Abuja in Nigeria.
The statement was issued in Arabic and French at a news briefing in the capital of neighbouring Chad, N'djamena.
"We have set up this union in the interests of the people of Darfur," Dr Ibrahim Khalil, president of the JEM, told reporters.
"To lose time without uniting our efforts means extending the days of the (Khartoum) regime which has become a factor in the disintegration of the regime."
"We have set up this union in the interests of the people of Darfur"
Dr Ibrahim Khalil,
The document was cosigned for the SLM by Mina Arko Minawi, a reputed hardliner, who last year displaced the founder-president of the SLM Abdelwahid Mohammed Nour at its head.
Both the SLM and the JEM said they opposed the choice of Sudan to head the African Union at its summit meeting next week. This pan-African gathering begins in Khartoum on Monday.
Fighting in Darfur began in February 2003 between rebel groups and the Khartoum government, supported by Janjawid militias. It is estimated to have cost some 300,000 lives and displaced more than two million refugees.