The 30-year-old joined the Red Devils on the final day of the winter transfer window on a deal that was supposed to run until May 31, 2020.
However, the Coronavirus pandemic has disrupted the footballing calendar around the world, leading to the suspension of the English Premier League.
Plans are in place to restart the EPL on June 17, meaning Ighalo’s previous deal at Old Trafford would have expired by then.
Ighalo has enjoyed his time in Manchester so far, scoring four goals in eight games before the season was halted.
A statement from United read: "The Nigerian striker's stay was initially due to end on May 31 but he will now remain with the Reds until Jan. 31 2021, enabling him to build on an impressive start to life with the club he supported as a boy.
"Today's confirmation is a timely boost for United boss Ole Gunnar Solskjaer, as he continues to prepare his squad for this month's resumption of competitive football."
“It’s been a dream for him and hopefully he can finish off what he started, with a trophy for us,” Solskjaer told MUTV.
Manchester United still have nine games remaining in the Premier League while they are still in the FA Cup and Europa League.
The new development means Ighalo will miss the start of the Chinese Super League which will resume next month.
