Dennis, 24, who had previously picked up just one red card in his career, during a 1-1 draw between Club Brugge and Gent, was given his marching orders by referee Mike Dean for a second bookable offence at Vicarage Road.
His first yellow card came before the break when Pierre Lees-Melou required treatment following a reckless challenge from the home side's leading scorer.
Canaries defender Max Aarons was then on the receiving end of another dangerous challenge committed by the two-time Belgian First Division A winner, who walked back down the tunnel in the 78th minute.
Dennis was excused from Africa Cup of Nations (AFCON) duty after the Nigerian Football Federation accused Watford of "baring fangs" after his nation missed the deadline to request his release.
He has been involved in 13 of the 23 goals scored by Claudio Ranieri's side in the top flight this term, scoring eight and adding five assists.
The summer signing scored 10 minutes into his debut in a 3-2 victory at home to Aston Villa and had netted the opening goal in the reverse fixture against the Canaries at Carrow Road.
The ex-Cologne man continued to have the Midas touch, finding the net against Everton and Manchester United, with Watford winning all four fixtures in which he scored.
However, their luck soon turned. Leicester City, Chelsea, Brentford and West Ham United were his next victims, though Watford would lose each of those, picking up a solitary point from eight games.
They're now in the bottom three, alongside Burnley and Newcastle United, two points adrift of safety and three points better off than the Clarets, who now have three games in hand.
“Norwich gave us a right lesson, they played as a squad, they played together, they fight together and they won," said Ranieri, following Friday night's damaging 3-0 loss.
"I asked the players to light their fire and the first half was so so, not good, not what I expected and the second half, after the goal we disappeared. Maybe we played better with 10 than 11."