The memorial – a circular pool of water with a small plinth in the middle – will be in the Tiergarten park, near the Reichstag, the German parliament building.
The unveiling comes after years of delays and disputes over the memorial’s design and its cost.
Experts say between 220,000 and 500,000 Roma were killed during World War II.
“It’s very important to me that we have a culture of remembrance,” German Chancellor Angela Merkel said in an interview on her YouTube channel.
“Every generation must confront its own history afresh. And for that we must have suitable places that people can go to in the future, when the witnesses from the time are no longer alive.”
Angela Merkel acknowledged that the building of the memorial had taken a long time and entailed “many discussions”, and recalled that the memorial to murdered Jews of Europe had also taken more than 15 years to complete.
President Joachim Gauck and some 100 elderly survivors will join Angela Merkel at the opening ceremony on Wednesday.
The memorial has been designed by the Israeli artist Dani Karavan. A fresh flower will be laid on the plinth at the centre of the memorial every day.
“Auschwitz” by Italian poet Santino Spinelli is engraved around the pool’s rim.
A chronology of the Nazi extermination campaign stands next to the memorial.
In 1982, Germany officially recognized the genocide of the Roma and Sinti – a related people who live mostly in German-speaking areas of Central Europe.
The leader of the Central Council of Sinti and Roma in Germany, Romani Rose, will also be at the ceremony.
He told Agence France-Presse: “Opening the memorial sends an important message to society that anti-Roma sentiment is as unacceptable as anti-Semitism.”
In recent years, Germany has been moving to commemorate the persecution of the Roma during World War II.
However, Roma organizations and human rights groups say they are still discriminated against in many European countries.
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