The king's speech on Saturday followed Dutch Prime Minister Mark Rutte's apology late last year for the country's role in the slave trade and slavery. It is part of a wider reckoning with colonial histories in the West that have been spurred in recent years by the Black Lives Matter movement.
In an emotional speech, Willem-Alexander referred back to that apology as he told a crowd of invited guests and onlookers: "Today I stand before you. Today, as your King and as a member of the government, I make this apology myself. And I feel the weight of the words in my heart and my soul."
The king said he has commissioned a study into the exact role of the royal House of Orange-Nassau in slavery in the Netherlands.
"But today, on this day of remembrance, I ask forgiveness for the clear failure to act in the face of this crime against humanity," he added.
Willem-Alexander's voice appeared to break with emotion as he completed his speech before laying a wreath at the country's national slavery monument in an Amsterdam park.
Former lawmaker John Leerdam told Dutch broadcaster NOS that he felt tears running down his cheeks as the king apologised.
"It's a historic moment and we have to realise that," he said.
Slavery was abolished in Suriname and the Dutch colonies in the Caribbean on July 1, 1863, but most of the enslaved labourers were forced to continue working on plantations for a further 10 years. Saturday's commemoration and speech mark the start of a year of events to mark the 150th anniversary of July 1, 1873.
Research published last month showed that the king's ancestors earned the modern-day equivalent of €545 million ($892 million) from slavery, including profits from shares that were effectively given to them as gifts.
When Rutte apologised in December, he stopped short of offering compensation to descendants of enslaved people.
Instead, the government is establishing a €200 million ($327 million) fund for initiatives that tackle the legacy of slavery in the Netherlands and its former colonies and to improve education about the issue.
That isn't enough for some in the Netherlands. Two groups, Black Manifesto and The Black Archives, organised a protest march before the king's speech Saturday under the banner "No healing without reparations."
"A lot of people including myself, my group, The Black Archives, and the Black Manifesto say that (an) apology is not enough. An apology should be tied to a form of repair and reparatory justice or reparations," said Black Archives director Mitchell Esajas.
Marchers wore colourful traditional clothing in a Surinamese celebration of the abolition of slavery. Enslaved people were banned from wearing shoes and colourful clothes, organisers said.
"Just as we remember our forefathers on this day, we also feel free, we can wear what we want, and we can show the rest of the world that we are free," said 72-year-old Regina Benescia-van Windt.
The Netherlands' often brutal colonial history has come under renewed and critical scrutiny in the aftermath of the killing of George Floyd, a Black man, in the US city of Minneapolis on May 25, 2020, and the Black Lives Matter movement.
A groundbreaking 2021 exhibition at the national museum of art and history took an unflinching look at slavery in Dutch colonies. In the same year, a report described the Dutch involvement in slavery as a crime against humanity and linked it to what the report described as ongoing institutional racism in the Netherlands.
The Dutch first became involved in the trans-Atlantic slave trade in the late 1500s and became a major trader in the mid-1600s. Eventually, the Dutch West India Company became the largest trans-Atlantic slave trader, according to Karwan Fatah-Black, an expert in Dutch colonial history and an assistant professor at Leiden University.
Authorities in the Netherlands aren't alone in saying sorry for historic abuses.
In 2018, Denmark apologised to Ghana, which it colonised from the mid-17th century to the mid-19th century. King Philippe of Belgium has expressed "deepest regrets" for abuses in Congo. In 1992, Pope John Paul II apologised for the church's role in slavery. Americans have had emotionally charged disputes over taking down statues of slaveholders in the South.
In April, King Charles III for the first time signalled support for research into the UK monarchy's ties to slavery after a document showed an ancestor with shares in a slave-trading company, a Buckingham Palace spokesperson said.
Charles and his eldest son, Prince William, have expressed their sorrow over slavery, but haven't acknowledged the crown's connections to the trade.
During a ceremony that marked Barbados becoming a republic two years ago, Charles referred to "the darkest days of our past and the appalling atrocity of slavery, which forever stains our history." English settlers used African slaves to turn the island into a wealthy sugar colony.
Willem-Alexander acknowledged that not everybody in the Netherlands supports apologies, but called for unity.
"There's no blueprint for the process of healing, reconciliation and recovery," he said. "Together, we are in uncharted territory. So let's support and guide each other."
