The incident happened in the Zaporizhia Region town of Melitopol.
Police said that the 28-year-old man had been sitting on a bus when another man got onboard and demanded that he give him his seat. After he refused, the man returned with a knife and stabbed the passenger in the chest.
The passenger, who has not yet been named, lost consciousness after the attack and died later in hospital. Police subsequently detained a 30-year-old man with previous convictions for robbery.