What is the main fat used in a traditional French croissant? Butter. Butter is the defining fat in the classic French-style croissant, where it creates the pastry’s rich flavor and delicate flaky layers.
Croissant dough is laminated by repeatedly rolling and folding dough around sheets of butter. During baking, water in the butter turns to steam, helping separate the layers and producing the pastry’s characteristic lift and honeycomb crumb. The butter must remain firm enough to form clean layers but pliable enough to roll without tearing the dough.
Commercial bakeries may use margarine or other specialized roll-in fats because they can be easier to handle across a wider temperature range. That does not change the traditional answer: butter is the standard fat associated with an authentic French croissant.
Croissant dough also contains flour, water or milk, yeast, sugar, and salt. It is not the same as puff pastry, because croissant dough is yeast-leavened. Lard and vegetable oil may appear in regional or industrial variations, but neither is the canonical fat for a traditional French croissant.