Henderson, who has committed to the Hornets until 2011, said: "It is nice finally to put pen to paper. I am delighted - hopefully I can carry on where I left off on Saturday (when Henderson netted the opener in the 2-0 win over Ipswich) by scoring goals.
"We have nine points out of a possible 12, so at the moment we are in promotion form.
"All of the speculation is in the past. I am just looking forward to giving my all for Watford."