These are official Splatoon socks. The perfect product description has already been written in Japanese, so we've included this below in direct translation:
"Reproduce sneakers that appear in the game with socks!
You can choose from three types of sea cucumber purple, sea eagle green, and sea urchin red.
Design that seems to be wearing shoes even when taking off shoes!"
You'd have to pull a 'G' prize ticket to win these from the Ichiban Kuji lottery in Japan, but as part of our set split you can now purchase them individually.