i thought so after posting reply. that is why i edited it...
anyway, the top side seem to be marked as cathode and connected to track marked 5V (if my eyes can be trusted).
most likely options could be Schottky diode for getting 5V from USB port or zener diode if anode was connected to GND.
it does not seem to be tied to GND because there is large ground plane right next to it. also track seem to go towards USB port...
unlike regular rectifier diodes, Schottky has low voltage drop and it is commonly used in such places to prevent backfeed
of power to USB when circuit is powered from external supply such as 12V (there seem to be 5V regulator AMS1117-5.0).
too large voltage drop (if using standard diode) would reduce "5V" rail for ICs below lower threshold so this is a job for Schottky
if the board was to be powered by USB only