The Hornets, recently promoted back to the Premier League at the first attempt, secured their third addition of the window having agreed to sign the Nigerian striker from Club Brugge.
Once personal terms are sorted, Dennis will join free agents Danny Rose and striker Ashley Fletcher at Vicarage Road.
He spent the second half of last season on loan in the Bundesliga with Cologne but scored just once in 10 appearances.
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The 23-year-old had been on loan from Brugge having spent most of his career in Belgium, scoring an impressive 29 goals in all competitions, two of which came in a Champions League group stage draw against Real Madrid in October 2019.
However, Dennis hardly featured during his final few months at the club and didn’t play a single minute in their last eight games prior to his January exit.
And Pinder told Football FanCast exactly why he left Brugge and then struggled in Germany:
“I think Club Brugge got rid of him for disciplinary reasons, but also coming to a club like Cologne that need goals doesn’t help because there’s a lot of pressure and it’s a new league. I just don’t think he settled.”