The 48-year-old man is a high-risk contact of the first patient, who died on April 21, said the WHO on Twitter.
The current outbreak is Congo's 14th in the history of hemorrhagic fever.
Health workers started Ebola vaccinations last week in Mbandaka, the capital of Congo's Equateur province.
Responders have identified 444 contacts of the confirmed cases and are monitoring their health, the WHO said.
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Congo's equatorial forests are a natural reservoir for the Ebola virus, which was discovered near the Ebola River in northern Congo in 1976.
The country has seen 13 previous Ebola outbreaks, including one in 2018-2020 in the east that killed nearly 2,300 people, the second-highest toll recorded in the history of the hemorrhagic fever.
The most recent ended in December in the east and caused six deaths. Mbandaka, the capital of Equateur province, has also contended with outbreaks in 2018 and in 2020.
Genetic testing has shown that the current outbreak was a new "spillover event", meaning it was transmitted from infected animals rather than linked to previous events.