Officials said turnout was estimated at more than 70%, with analysts predicting that the king will win the vote.
His reforms come after street rallies inspired by the so-called Arab Spring uprisings, which ousted leaders in Tunisia and Egypt.
'Date with history'
Polls closed at 1900 (1800 GMT), with preliminary results due shortly afterwards.
Officials said that turnout - based on the count from two-thirds of polling stations across the country - was 70.6%.
The king did not say anything as he cast his vote in a chic district of the capital, Rabat.
The vote, which represents the first constitutional referendum under the king's 12-year rule, has been described by one Moroccan newspaper as "a date with history".
Morocco's own youth-based February 20 Movement organised weeks of pro-reform demonstrations through websites such as Facebook and YouTube which brought thousands on to the streets. They urged their supporters to boycott the vote.
At the sceneSome call it the "big decision day" here in Morocco. Yet Moroccans seem to be going about their daily life without much fuss unlike the days leading up to the referendum. Some people here say it is the quiet before the storm.
In one of the polling centres, set up in Hassan II High School in the capital, Rabat, all voting rooms looked the same. They contained a Moroccan flag and a poster of the king, both taped next to each other on the blackboard. Participation was slow in the morning but people say much more voters will turn out after Friday prayers.
There is a sense of anticipation in the kingdom, many people feel they are making history and they say they are weighing their vote carefully. They know their vote - no or yes - may represent a referendum on the king himself.
The king himself has described the reforms as: "A decisive historic transition."
"I support the king, he keeps Morocco safe. It is not like Algeria and Yemen, it's stable here," Rachid Aboul-Hassan, a cab driver in the capital, Rabat, told the AP news agency.
"There are problems here, but we are taking small steps, slowly."
Under the draft constitution, the king remains as the head of state, the military, and the Islamic faith in Morocco, but the prime minister - to be chosen from the largest party elected to parliament - would take over as head of the government.
The reforms, the king has pledged, would reinforce the independence of the judiciary, boost efforts to tackle corruption, guarantee freedom of expression and gender rights and make Berber an official language, alongside Arabic.
"The majority will approve the reform. What's really at stake is voter turnout," said Lahcen Daodi of the moderate Islamist Justice and Development opposition party (PJD), which supports the reform.
The reform plan has been welcomed abroad, with the European Union saying it "signals a clear commitment to democracy".
But it fails to meet the demands of a full constitutional monarchy sought by many protesters.
Many activists have been sceptical about the king's promises of change, saying Morocco's 400-year-old monarchy has a long history of enacting superficial reforms.
Morocco has been facing severe economic challenges with high unemployment and rising levels of poverty.
King Mohammed, 47, acceded to the throne in 1999 following the death of his father, Hassan II, and now heads the Arab world's longest-serving dynasty.