The party reached 26.8 percent in the latest poll by Index Research, 0.3 percent up on last week, while the ruling Democratic Party stood at 25.1 percent.
Di Maio has distanced himself from the eurosceptic politics of the party’s founder Beppe Grillo, saying earlier this month that “we do not want a populist, extremist or anti-European Italy”.
After being elected on Sunday he said the party wants to stay in the EU, but would endeavour to “change a number of treaties that are hurting our economy and businesses”.
Meanwhile, the centre-right Forza Italia scored 14 percent in the poll, with the far-right Northern League standing at 14.8 percent.
The research firm said that if all the centre-right parties merged they would score 34.4 percent, making them the top coalition.
The Five Star Movement has a strong presence in parliament and scored mayoral victories in Rome and Turin last year.