Oh, this is a weirdly specific bit of lore that trips people up sometimes. He was definitely a Gryffindor—it's all over the books and supplementary stuff. But I think what makes the question pop up is that we don't get the actual Sorting scene for him, not like we do for Harry. We just have to infer it from everything else. He's in all the old photos with James and Lupin and Pettigrew in their Gryffindor scarves, and he talks about the common room like it's home. The fact that he's so strongly associated with the whole Marauders-as-Gryffindors idea makes it pretty airtight.
That said, I get why someone might wonder. He's got a lot of Slytherin family baggage, and he can be cunning and ruthless when he needs to be. The whole framing of his escape from Azkaban was a masterstroke of planning, which feels a bit non-Gryffindor. But then, the Sorting Hat probably saw his core defiance, his willingness to stand up to his whole awful family, and his loyalty to James. That's bravery in its rawest form, even if it's messy. The hat doesn't always put you where you come from; it puts you where you value. Sirius valued bravery and rebellion above all else, even if it got him killed.
So yeah, 100% Gryffindor. It's almost the most important thing about his character—he chose to be something different than what his bloodline demanded. The house wasn't just a label; it was his first real act of self-determination. Kind of poetic, really.