Leaders of the Economic Community of West African States had decided to act to "take care of our own security in the region", Omar Alieu Touray, president of the ECOWAS commission, told journalists at a summit in Nigeria.
They are "determined to establish a regional force that will intervene in the event of need, whether this is in the area of security, terrorism and restore constitutional order in member countries," he added.
Mali, Guinea and Burkina Faso have all been hit by military coups in the last two years.
Several countries in the region are also suffering from the spread of jihadism, including Mali, Burkina Faso and Niger, and southwards to the Gulf of Guinea.
National armies, largely powerless against the jihadist forces operating across borders, have been cooperating with external actors such as the UN, France and Russia.
But Touray said this decision would "restructure our security architecture".
The modalities of the planned regional force will be considered by defense chiefs in the second half of January, 2023, Touray said.
The funding of the force must also be decided, but the ECOWAS official stressed that such an operation could not be solely dependent on voluntary contributions.
The West African leaders, concerned about instability and contagion, have been pressing for months for the quickest possible return to civilian rule in the three countries which have undergone coups in recent years.
Mali and Burkina Faso have both been severely shaken by the spread of jihadism.
All three countries have been suspended from the decision-making bodies of ECOWAS.
Leaders of the military juntas have pledged, under pressure, to step down after two years, allowing for a transition period during which they all say they want to "rebuild" their state.
ECOWAS has been looking to see what progress each nation has been making towards restoring constitutional order.
In Mali, "it is essential that constitutional order returns within the planned timeframe", said Touray.
If Mali's military meets the announced deadline of March 2024 -- after months of confrontation with ECOWAS and a severe trade and financial embargo that has now been lifted -- the "transition" will in fact have lasted three and a half years.
Touray urged the junta in Guinea to involve all parties and civil society in dialogue "immediately" on the process of restoring civilian rule.
The main political parties and much of civil society there have been boycotting the authorities' offer of dialogue.
As for Burkina Faso, Touray expressed ECOWAS's "serious concerns" about the security situation and the humanitarian crisis there, while pledging support for the country.
Addressing another regional problem, the West African leaders told Mali's ruling junta to release 46 Ivorian troops it has held since July.
"We ask the Malian authorities to release the Ivorian soldiers by January 1, 2023 at the latest," said Touray, at the Abuja summit.
The Gambian diplomat said the West African bloc reserved the right to act if the soldiers were not released by January 1.
If Mali fails to do so, ECOWAS will impose sanctions, a West African diplomat told AFP.