Macquarie's new Japanese subsidiary will be renamed Acacia Renewables and will develop the current firm's portfolio of onshore wind.
RES Group will continue to offer advisory and technical services to Acacia.
"Macquarie is one of the world's largest investors in infrastructure and renewable energy, and we are confident that with Macquarie's support and the in house expertise of the Acacia Renewables team, the business will continue to grow into a leading renewable energy company in Japan," said Acacia CEO David Povall.
According to the RES Group website, its portfolio in Japan comprises just one 19.95MW project in Akita, northwest Japan.
The deal follows Macquarie's acquisition of the UK's Green Investment Bank in April for €2.75 billion.