The dance teacher from Edinburgh spat in fellow housemate Mohamed Mohamed's face during an argument last night.
After reviewing the footage, Channel 4 bosses called Dennis, 23, to the diary room where he was immediately told to leave.
'Spitting in someone's face is seriously offensive and Mohamed was understandably upset', a Channel 4 spokesman said.
'Big Brother has very clear rules about offensive behaviour.
'Like all housemates, Dennis was clearly informed before entering the house that he faced eviction if he acted in an unacceptable manner'.
The incident happened after Rex Newmark, 24, smudged Jennifer Clark's portrait of new boy Stuart Pilkinton, 25, as a 'joke'.
But it backfired when Jen, 21, started crying and Mohamed, 24, called her an 'attention seeker'; prompting a reaction from Dennis.
All the housemates have been questioned over the incident.
Tonight's eviction between Mohamed and Sylvia will go ahead as planned.
To see the moment the house found out about Dennis leaving, click on the link below: