The incident occurred on Wednesday during flight AI2336 from New Delhi to Bangkok when the 24-year-old Indian man, reportedly intoxicated, stood up and urinated on the man seated next to him, according to NDTV.
Cabin crew responded promptly, providing towels for cleaning and escorting the Japanese passenger to the lavatory to change clothes.
The offender was relocated to another seat.
Crew members also offered to assist the affected passenger in filing a formal complaint with authorities in Bangkok, but he declined, saying he did not want to waste time after landing, The Indian Express reported.
Air India later announced that an independent committee would review the incident and determine whether further action is necessary.