It has been reported by the British tabloid that the Premier League outfit are looking at the striker as a potential signing at the end of the season.
“The Nigeria international turned down a massive offer to play in China last summer but could move this summer for £15million,” added the report in The Sun.
Ighalo has been on the books of Watford since 2014 and has a contract until June 2020.
The Nigeria international striker has scored 14 goals and created 35 chances in 31 Premier League appearances for the Hornets so far this season.
The Watford man has a pass accuracy of 75% and a shot accuracy of 57%, and has won 14 headers, 40 dribbles and 17 tackles.
Leaving Watford and joining West Ham would be a step-up for Ighalo, but it remains to be seen if the Hornets are prepared to part company with the former Udinese striker in the summer transfer window.
West Ham are fifth in the Premier League table at the moment, while Watford are currently 14th in the standings.