Guinea-Bissau's election commission has confirmed a presidential run-off vote will take place on May 18.
Former finance minister Jose Mario Vaz will face Nuno Gomes Nabiam, an independent candidate who headed the civil aviation authority.
Vaz, who is backed by PAIGC, the country's largest political party, received just over 40 percent of the vote in first-round voting April 13. Nabiam, who is seen as the military's candidate, garnered 25 percent.
The election was the first in Guinea-Bissau since a 2012 military coup disrupted a presidential poll as it headed to a second round.
Guinea-Bissau is one of Africa's smallest and most unstable countries. The country has endured a series of coups, mutinies and political assassinations since independence from Portugal 40 years ago.
No elected president has ever finished his mandate.