Historians say the Nazis exterminated nearly 500,000 Roma men, women and children in Europe during World War II, decimating a population that had roots in Germany dating back six centuries.
The memorial, given pride of place in Berlin's central Tiergarten park between the Reichstag parliament building and the Brandenburg Gate, will be unveiled after years of delays and bitter disputes over its design and cost.
"A culture of remembering is very important to me," said Merkel, who will also be joined at the opening by President Joachim Gauck and about 100 survivors, in her weekly video podcast.
"That is why we must have appropriate places where that is possible - where people can also go in the future when the survivors are no longer alive."
The monument, whose construction began in December 2008, consists of a round pool of water with a stele in the centre on which a single fresh flower will rest each day.
A time-line of Nazi extermination campaigns stands next to the memorial, which was built at a cost of €2.8 million.
It was designed by Israeli artist Dani Karavan, 81, and is located near other memorials for victims of Nazi barbarism, including a sprawling field of stones in honour of the six million murdered Jews and a smaller monument for homosexual victims.
The opening has been accompanied by a series of concerts, art exhibitions, public debates and readings.
The Roma and related Sinti, like the Jews deemed racially inferior in Nazi ideology, were also systematically targeted by the Nazis, confined to ghettos and special camps, deported and killed.
In 1938, SS chief Heinrich Himmler ordered the "final solution of the gypsy question".
In the service of the genocidal programme, the "racial scientists" of Nazi Germany developed a list of characteristics and traced family trees in some cases back to the 16th century to discover "gypsy ancestors".
Those caught in the sweeping campaign were sent to concentration camps.
At Auschwitz and Ravensbrück, they were subjected to grotesque medical dxperiments. But West Germany did not recognise the genocide until 1982.
"The Holocaust against the Roma - or 'Porajmos' which literally means 'devouring' - was long denied and was not the subject of historical research, not only in Germany, but also in other countries such as Vichy France or countries of eastern Europe that participated in the persecution," historian Wolfgang Wippermann of Berlin's Free University said.
The government's decision to erect the monument dates back to 1992, but it was held up in part by a semantic dispute.
The head of the Central Council of Sinti and Roma in Germany, Romani Rose, objected to the memorial referring to "gypsies", a term commonly used in the past but now viewed as derogatory.
"Unlike the Jews, whom the Nazis found by their religion, Roma - most of them Catholics - were not necessarily distinguishable from other citizens," Rose, who leads a community now numbering about 70,000, told AFP.
Some 11 million Roma now live in Europe, seven million in the European Union, but they still suffer from disproportionate poverty and rampant discrimination.
The fall of the Berlin Wall in 1989 sent many fleeing south-eastern Europe for the richer west and countries such as France and Italy have in recent years cracked down on illegal camps.
Germany has indicated it would like to curb the flow of Roma into the country, possibly by reinstating visa requirements for Serbia and Macedonia.
The secretary general of the Council of Europe, Thorbjørn Jagland, slammed the enduring persecution faced by Roma today, calling it one of the continent's greatest challenges.
"The biggest minority in Europe is discriminated against in most European states and the Roma's living conditions are horrible in many countries," he told Sunday's issue of the Tagesspiegel newspaper.