Faced with protests modelled on the Arab Spring uprisings that ousted long-serving leaders in Tunisia and Egypt, Mohammed VI announced the referendum last month to devolve some of his wide-ranging powers to the prime minister and parliament of the north African country.
Under a draft constitution to be voted on Friday, the king would remain head of state, the military, and the Islamic faith in Morocco, but the prime minister, chosen from the largest party elected to parliament, would take over as head of the government.
Analysts say there is little doubt voters will approve the new constitution, with the only question whether turnout will be high enough to ensure the referendum result's credibility.
Mohammed VI, who in 1999 took over the Arab world's longest-serving dynasty, offered the reforms after the youth-based February 20 Movement organised weeks of pro-reform protests that brought thousands to the streets.
The reforms fall short of the full constitutional monarchy many protesters were demanding and the movement has urged its supporters to boycott Friday's vote.
The reform plan has been hailed abroad, however, with the European Union saying it "signals a clear commitment to democracy".
Throughout a brief campaign, the new constitution has been fiercely backed by the country's main political parties, unions, civic groups, religious leaders and media. The campaign was dominated by the "yes" side, with few signs of an organised "no" vote movement.
The February 20 movement has continued to hold protests, organised through websites such as Facebook and YouTube, since the reforms were announced and maintains they do not go far enough.
Along with changes granting the prime minister more executive authority, the new constitution would reinforce the independence of the judiciary and enlarge parliament's role.
It would also remove a reference to the king as "sacred", though he would remain "Commander of the Faithful" and "inviolable".
The new constitution would also make Berber an official language along with Arabic -- the first time a North African country has granted official status to the region's indigenous language.
Voting was to start in 40,000 polling stations across the country at 8:00 am local time (0700 GMT; 11 am UAE time) and the polls close at 7:00 pm (1800 GMT). Preliminary results are expected late Friday or early Saturday.