The survey by Forsa for broadcaster RTL showed support for the Pirates, whose platform is based on internet freedom and more direct participation in politics, pushing up to 13 percent and outstripping the Greens for the first time.
An off-shoot of a party that was founded in Sweden six years ago, the German Pirates came out of nowhere last September to win seats in the city government in Berlin.
At first dismissed as a passing fad by the established parties, the Pirates followed up their success in Berlin with a strong showing in the state of Saarland last month and now look on track to make it into regional assemblies in two other states -- North Rhine-Westphalia and Schleswig Holstein -- next month.
Their rise has thrown the Greens in particular onto the defensive, threatening their hold on younger voters disillusioned with the two big parties -- the Christian Democrats (CDU) and Social Democrats (SPD) -- that have dominated post-war politics in Germany.
The Greens, who rose to prominence in the 1980s on a pacifist, anti-nuclear platform, are now struggling to differentiate themselves from the big established parties after CDU Chancellor Angela Merkel dropped her support for nuclear power last year following the Fukushima disaster in Japan.
“For many young people, the Greens have become an old party. The anti-nuclear theme just doesn't lure voters like it used to,” said Manfred Guellner, director of Forsa.
The poll showed the Greens on 11 percent, just half the levels they were scoring last year in the immediate aftermath of Fukushima. The SPD stood at 24 percent and the far-left Left party on 8 percent.
The survey showed the Free Democrats (FDP), a business-friendly party whose support has collapsed over the past year amid policy missteps and infighting, back at the 5 percent level needed to enter parliament for the first time in nearly a year.
The FDP, junior partners to Merkel's CDU in the federal government in Berlin, may have lured back some traditional free-market supporters with their recent decision to veto state aid for sacked employees of bankrupt drug store firm Schlecker.
The return of the FDP above the 5 percent threshold and the rise of the Pirates has raised the prospect of six party groupings -- the CDU/CSU, SPD, Pirates, Greens, Left and FDP -- regularly making it into German parliaments at the state and national level.
That would further complicate coalition-building and could increase the likelihood of so-called “grand coalitions” of the CDU and SPD being formed.
Merkel presided over such a coalition during her first term between 2005 and 2009, and a similar partnership of the center-right and center-left is seen by many as the most likely outcome of the next federal vote in 2013.
The Forsa survey put Merkel's CDU and its Bavarian sister party, the Christian Social Union (CSU), on a combined 36 percent.