In Friday’s post on social media, Zelensky wrote: “At this time, Ukraine needs greater focus on security issues, the development of the defence and security forces of Ukraine, as well as on the diplomatic track of negotiations.
“Kyrylo has specialist experience in these areas and sufficient strength to deliver results.”
The president added that he had already instructed his new office chief to update and present key documents regarding “the strategic foundations” of Ukraine’s defence.
Yermak, 54, stepped down on 28 November, and his departure was seen as a major blow to Zelensky.
Yermak quit shortly after his home in Kyiv was raided by the country’s anti-corruption agencies.
He is not accused of any wrongdoing, and the anti-corruption bureau Nabu and specialised anti-corruption prosecutor’s office Sapo did not explain why they searched his property.
In the past few months investigators have linked several high-profile figures to an alleged $100m (£75m) embezzlement scandal in the energy sector.
They said they had uncovered an extensive scheme to take kickbacks and influence state-owned companies including state nuclear energy firm Enerhoatom.
The corruption scandal has rocked Ukraine, weakening Zelensky’s own position and jeopardising the country’s negotiating position at a delicate time.
Kyiv, backed by its European allies, is seeking to change the terms of a US-led draft peace plan originally seen as heavily slanted towards Russia.
Russian officials have seized on the scandal, talking up corruption claims.