About 70 soldiers from Burkina Faso were deployed in Guinea-Bissau on Thursday, the advance party of a 600-strong West African force that is due to replace Angolan troops and oversee a transition back to civilian rule.
Guinea-Bissau was just weeks away from holding a presidential run-off vote when a military junta took over.
The ECOWAS soldiers arrived on the same day as the prime minister of a transitional government, Rui Duarte Barros, was sworn into office.
The coup leaders had earlier agreed to a 12-month transition to civilian rule, as demanded by ECOWAS.
The West African force is due to oversee that transition and help push through reforms of the army, which has long meddled in politics and, in recent years, has been widely accused of facilitating the drugs trade.
The ECOWAS peacekeepers are being deployed to replace an Angolan mission, which had been in the country for about a year, but which fell out with local military officers.