Bullard's exact misdemeanour remains unclear, although it has been suggested the incident occurred during the Championship club's training camp in Slovenia last week.
He is believed to have returned to England with his team-mates before being ordered to stay away from City's training ground for two weeks.
The 32-year-old was omitted from Hull's 2-1 pre-season victory at non-league Winterton Rangers on Wednesday night.
Bullard has two years remaining on his £45,000-a-week contract at the KC Stadium and has repeatedly been linked with a move away from the club over the past 12 months, spending the second half of last season on loan at Ipswich.
The Tigers said in a statement said: "Hull City AFC can confirm that Jimmy Bullard has been suspended pending an inquiry into recent events.
"The club will be completing the investigation before making any further comment."