A largely unknown party, rather indifferent to European public opinion and the media until 2009, SYRIZA constitutes probably an exception among the family of the European radical left, as much by its rapid electoral ascent and its victory in the elections of 2015 as by the series of paradoxical choices it made once it had acceded to power. SYRIZA managed to capitalise on the structure of political opportunity created by the Greek financial crisis and on the gap created by the collapse of the prevailing two-party system and the electoral debacle of one of its two pillars during the 2010-2012 period; to master and promote an alternative narrative to the crisis, dominated by populism and the denunciation of the system, arguing that there was an alternative to austerity within the euro area; to conquer a sizeable part of the voters of the Greek Socialist Movement (PASOK) and New Democracy (ND), following in its rhetoric a strategy of rupture with the past, but of continuity and rallying in terms of political personnel and tactics. SYRIZA did not hesitate to resort to the institutional possibilities within its reach. By refusing to support the election of the President of the Republic under the ND-PASOK government in late 2014, it accelerated its rise to power. And it inevitably succeeded, then facing the responsibilities inherent in the exercise of power in the times of memoranda. Based on the hypothesis that the crisis acted as an external shock that awakened the dynamics of hitherto latent changes within the party, this paper studies the profound mutation of SYRIZA in terms of organisation, electoral sociology, discourse, policy-making, and coalition strategy. The present study contributes to the debate on the change of contemporary political parties linked to external constraints, as well as to the consequences that come with governance under austerity.