The 30-year-old's initial loan contract expired at the end of May, but the Red Devils have now reached an agreement with his parent club Shanghai Shenhua to keep him at Old Trafford until January 31, 2021.
The new terms do not include an option to buy him at the end of that period, suggesting that United could still be in the market for a permanent striker when the transfer window reopens.
Ighalo's extension enables him to see out the remainder of the 2019-20 campaign and first half of the 2020-21 season with United after they were given the green light to return to action this month.
The precise fixture list is yet to be announced, but United's first game back will be against Tottenham Hotspur on the weekend of June 19-21.
Ighalo has scored four times in eight games for United since joining from Shanghai on deadline day of the January window.