A fuel fill opening structure for a fuel tank includes a fuel filler pipe, a fuel fill opening, and a flow guide. The flow guide includes a main guide wall which partitions a fuel introduction region inside the fuel filler pipe into an upper space communicating with a tank main body side and a lower space communicating with the fuel fill opening side, and a partition wall provided to be continuous with the main guide wall and configured to close a space between an upper surface side of the main guide wall and an inner surface of the fuel filler pipe. The main guide wall includes a bent inclined part that is bent and inclined downward from a middle portion in running from the fuel fill opening side toward the tank main body side. The bent inclined part has a shape in which a width thereof becomes smaller when viewed from above as it inclines downward.