I found this seed on the ground in a park in Barcelona, Spain. I thought it might be a buckeye but it looks just different enough that it seemed possible that it might be something else. It's roughly 1.5 inches long at its widest point.
This is very likely Aesculus hippocastanum alias European Horse Chestnut. The seed/fruit presented in the picture is the result of drying of the fresh seed into the object known as a "conker" in the UK. Once they get to that condition they are not much use as seeds to produce new plants. The wiki page has a picture of the fresh seeds and their bright warm brown colour. Spain is of course also famous for sweet chestnuts but the classic difference in seeds is that the sweet chestnut produces multiple seeds in the same spiky fruit so they have a flat side; this one was completely round when fresh. We would need a picture of the tree to be more sure of the ID.
A search on "aesculus barcelona" produces links that show A. hippocastanum is grown and used in the area, and that it has been crossed with A. glabra (the related Ohio Buckeye) and other species to produce interesting hybrids.