The 18-year-old made his full debut for the Hornets in the draw at Swansea before the international break.
Lansbury, who is on loan from Arsenal until December 31, believes the move is a "step up" in his career.
He told the club's official website: "Playing at Watford will be another great learning curve in my career and playing in the Championship is a great experience for any young player.
"I was lucky enough to have a great loan period with Scunthorpe in League One last season and they were promoted to the Championship while I was there.
"Being at Watford is definitely a step up and a move forwards for me in my career so far so I hope I do well here."