The deal to bring the 30-year-old Nigeria international to Old Trafford contains no option to buy.
“Odion is an experienced player,” said United manager Ole Gunnar Solskjaer.
“He will come in and give us an option of a different type of centre-forward for the short spell he’s staying with us.
“A great lad and very professional, he will make the most of his time here.”
Ighalo moved to the Chinese Super League in 2017, first with Changchun Yatai. After two seasons he moved to Shanghai Shenhua and has scored 10 times in 19 games.
His form for his country has been eye-catching, finishing as top scorer in African Cup of Nations qualifying with seven goals, before scoring five at the tournament during the summer.
It is understood there was interest from other Premier League clubs but Ighalo wanted to play for United.
He is United’s first senior loan signing since Radamel Falcao’s arrival from Monaco in 2014.
Earlier on Friday, United ended interest in Bournemouth’s Josh King but did sign Southend United goalkeeper Nathan Bishop for an undisclosed fee.
United accept Ighalo does not fit the club’s policy of trying to sign long-term targets, such as Bruno Fernandes, who moved to Old Trafford from Sporting Lisbon on Thursday.
However, this move has been done to cover the short-term injury to Marcus Rashford and they could not go the rest of the season without bringing in a replacement.
England international Rashford, 22, is out with a stress fracture in his back sustained after coming on as a substitute during the FA Cup third-round replay win against Wolves on 15 January.
Back in 2015 after Watford’s promotion from the Championship, Ighalo was asked where he was most looking forward to playing in his first Premier League season.
No hesitation: “Old Trafford. It’ll be a dream come true to play against Manchester United, the team I supported in my youth”. Little did he know what would happen almost five years later.
Odion does a huge amount of charity work for people in Nigeria and he’s also still a very talented striker.
It’s a left-field move by United and I’m also sure that you’ll have read and heard all the negatives so to balance things out let me give you some positives: Ighalo is a natural number nine and a natural goalscorer with pace and power.
Barcelona tried to sign him just 12 months ago because they wanted that type of player. He turned them down.
He is a very strong character, too. The Nigerian got a lot of criticism back home after missing chances when the Super Eagles were knocked out by Argentina at the last World Cup (including abuse aimed at his family) but he came back stronger to finish top scorer not only in Africa Cup of Nations qualifying but also at the tournament itself, ahead of the likes of Sadio Mane, Mohamed Salah and Riyad Mahrez.
Ighalo’s proven in the Premier League and will be able to hit the ground running, fitness permitting.
He’ll also be a very popular team-mate who will give his all to be a success at United. Nobody will work harder.
It’s almost like a homecoming, too, as Ighalo’s family has been based in Manchester.
It’s a fairytale move for the United fan from Lagos. Now let’s see if, not for the first time in his career, he can prove the doubters wrong.