Ighalo is joining United on loan for the rest of the season but there is no option to buy in the deal.
United, which finally signed Bruno Fernandes for a fee of £46.5 million ($90 million) and a potential further £21.1 million ($40.7 million)) in add-ons, was eager to bring in a striker prior to the end of the transfer window.
With Marcus Rashford out injured, United was said to have had an approach for Josh King rebuffed by Bournemouth before turning its attention to Ighalo.
The Nigeria forward has spent the past three seasons in the Chinese Super League, scoring 10 league goals for Shanghai Shenhua last term after two prolific campaigns with Changchun Yatai.
But the 30-year-old will now bolster Ole Gunnar Solskjaer's attack, with United just managing to get a short-term loan deal completed before the deadline.
An official announcement from United will come later, but it is understood all aspects of the deal have been agreed.
Ighalo has Premier League experience, having scored 16 goals for Watford in 55 top-flight appearances between 2015 and 2017.