The Nigerian striker scored 39 goals in 100 appearances in two and a half years at Vicarage Road, including 17 goals in Watford’s first season back in the top flight after securing promotion in 2014/15.
That brought Ighalo, who originally joined Watford on loan in July 2014, a new five-year contract last August.
He has been off the boil this season, however, scoring just once in the Premier League. He has not scored in all competitions for 15 games.
The Nigerian joins a list of top players joining the Chinese soccer gold rush, with Chelsea’s Brazilian midfielder Oscar joining Shanghai SIPG this month for a reported 60 million euros. ($64.79 million).