The Norwegian striker appeared to strike Bristol City's Marlon Pack in Hull's 2-1 victory over the Robins in the EFL Cup and has been found guilty by the Football Association.
Diomande will now be suspended for three matches and will miss Hull's Premier League clashes against Watford, Southampton and Sunderland.
The incident was missed by the referee on the night, but the Hull man was given a retrospective punishment.
Diomande was expected to start Hull's match against Watford and is likely to be replaced by Will Keane.