Istilqal won 52 out of 352 parliamentary seats, despite widespread expectations against a backdrop of advances for Islamic movements worldwide in recent years that the Justice and Development (PJD) party would come out on top.
In the event, the PJD won just 47 seats in the lower house of parliament in Friday's voting, Chakib Benmussa said.
The results are likely to relieve tensions in Washington, which can now rely on a continuity of policy in a leading Muslim state.
Underscoring the decorative nature of the elections, the final word, as always, rests with the country's monarch, King Mohamed VI, who will name the next prime minister, ostensibly on the basis of the results.
The prime minister will then name a government, likely to include the PJD for the first time.
The elections were the second parliamentary polls since King Mohammed VI took power eight years ago.
Morocco has weathered the storm of America's war against Islamic terrorism relatively unscathed. It is a moderate Muslim nation that attracts hordes of Western tourists. But it has had its share of terrorist atrocities, including suicide attacks in Casablanca in 2003 and in 2007.
After it emerged that the bombers implicated in the deadly train attacks in Madrid in 2004 came from Morocco, the government instituted a wide-ranging crackdown that has tarnished the country's relatively democratic image.
Earlier this week, Reporters without Borders sent a letter to King Mohammed, who calls himself a "citizen-king," blaming him for restricting press freedom.
However, voters in this election appeared most concerned with the more mundane aspects of life, such as poverty and unemployment, voting out of economic rather than religious motives.
Some 5 million out of Morocco's 33 million citizens live on less than $2 a day, the World Bank has said.
The Socialist Union of Popular Forces, or USFP, a center-left party that won the last elections in 2002 and ruled together with Istiqlal, dropped to fifth place with 36 seats. The centrist Popular Movement and RNI parties were in third and fourth, with 43 and 38 seats.
Twenty-three parties in all, along with five independents, will serve in the new parliament.
