The England Under-21 international has signed for the Hornets on a five-year deal for an undisclosed fee.
Hughes broke onto the scene at Derby as a 16-year-old back in 2011, and has gone on to play 190 games for the Rams.
Hughes makes Premier League move
The midfielder has been on the radar of several Premier League clubs for a number of seasons now, and the youngster will have his first taste of top-flight football this coming season.
Hughes has impressed with his form for his country as well, he was called up to the Under-17 squad for the first time in 2012 – he played three games in the Algarve Tournament.
He is the second youngest player to be capped for the Under-21s, behind Theo Walcott, and was part of the side that recently lost to Germany at the European Under-21 Championships in Poland.
Deeney backs signing of midfielder
Hughes is the first signing under new Watford manager Marco Silva, who replaced former boss Walter Mazzarri in the summer.
The signing has been given the praise of captain Troy Deeney, who said he has “always rated” the midfielder on social media.
His tweet read: “I like this signing always rated Will very clever player what do you guys think??”
The fee is believed to be in the region of £5 million, although nothing was confirmed, and it may even rise to £9 million because of add-ons.
Deeney was in discussion with several football fans on Twitter, and the striker agreed that the fee for Hughes is a “steal” in the current market.
The skipper was also quick to get behind the signing and responding to some fans that thought Watford shouldn’t be looking to sign Championship players. Deeney told those fans that they “should watch more games” if they have those views.