"I swear that I shall defend and preserve the independence and territory of the state with all my powers," he said.
"That I shall protect the general and individual freedom and rights of all my subjects and shall use all available means granted to me by law for preserving and promoting general and individual prosperity as I befitting of a good king.... So help me God almighty."
Willem-Alexander - who is a water management specialist, a useful expertise in a country where much of the land is below sea level - is expected to bring a less formal touch to the monarchy at a time of national austerity and budget cuts.
April 30, or Queen's Day, has always been an occasion for partying in the Netherlands, and Amsterdam has been awash with orange - the color of the House of Orange - for days.
An estimated 25,000 people, many dressed in orange or wearing orange wigs, hats, feather boas and crowns, massed in Dam Square next to the Royal Palace to watch the abdication and inauguration being broadcast live.
The queen had announced her intention to stand down in January, saying her son was ready to reign and that it was time for the throne to be held by "a new generation".
She formally relinquished the throne at a short ceremony in the Royal Palace on Tuesday, signing a statement transferring the monarchy to her son "in accordance with the statutes and the constitution of the Kingdom of the Netherlands".
Huge cheers erupted in Dam Square, which is situated next to the palace, from those watching the ceremony on giant television screens, as she, her son and his wife Maxima signed the deed of abdication.
Shortly afterwards, the three royals emerged on a balcony above the square.
The visibly emotional Princess Beatrix told the crowds: "I am happy and grateful to introduce to you your new king, Willem-Alexander."
Before taking his oath, the king thanked his mother for "33 moving and interesting years", saying he and the public and people in Dutch overseas territories were "intensely grateful" to her.
King Willem-Alexander was inaugurated in a decommissioned church, before a joint session of the Dutch parliament.
At his investiture in Amsterdam's 600-year-old Nieuwe Kerk, or New Church, the 46-year-old monarch swore an oath to uphold the Dutch constitution and stressed the need for unity at a time of economic crisis.
"I take office in a period when many in the kingdom feel vulnerable or uncertain. Vulnerable in their job or in their health, uncertain about their income or their immediate environment," Willem-Alexander said at his inauguration, attended by crown princes and princesses and other dignitaries.
"We can no longer take it for granted that children will be better off than their parents ... Our strength is therefore not in isolation but by cooperating."
The three then held hands as the national anthem was played, before the new king and the queen's three young daughters were brought out to wave at the crowds.
Their eldest daughter, nine-year-old Catharina-Amalia, has become Princess of Orange and is now first-in-line to the throne.
Many international royals and high-ranking dignitaries are taking part in the events, including the UK's Prince Charles and the Duchess of Cornwall, Prince Felipe and Princess Letizia of Spain and Denmark's Crown Prince Frederik and his wife.
Willem-Alexander has said he wants to "be a king that can bring society together, representative and encouraging in the 21st Century".
He has said he does not expect to be called "his majesty," saying people can address him "as they wish".
He is the seventh monarch from the House of Orange-Nassau, which has ruled the Netherlands since the early 19th Century.
Under Dutch law, the monarch has few powers and the role is considered ceremonial.
He or she is expected to be politically impartial, co-sign acts of parliament, help with the formation of new governments and to undertake state visits.
King Willem-Alexander has become not only the monarch of the Netherlands but also the Dutch Caribbean territories of Curacao, Aruba and Sint Maarten. He holds several military titles but requested an honourable discharge before his accession.
King Willem-Alexander was inaugurated in a decommissioned church, before a joint session of the Dutch parliament. (picture Getty images)