The reality TV star - who is rumoured to be dating the One Direction singer - arrived in the UK over the weekend, where she is said to be teaming up with Harry's friend, radio DJ Nick Grimshaw, to organise a wild celebration at The Box nightclub in London.
A source told the Daily Mail newspaper: ''They are throwing Harry a belated 20th birthday party.
''It's a sign of how far their relationship has come that she and Nick, one of Harry's closest friends, are doing it together. They have many surprises planned.''
'Story of my Life' star Harry turned 20 on February 1 while in California, where he had a low key day celebrating with Kendall, 18, at her father, Bruce Jenner's mansion in Malibu.
The singer's 19th birthday was a wild affair, with him partying with Nick at trendy London club Alibi where they ran up a £5,000 bar tab and Harry received a lap dance from a stripper dressed as a policewoman.
A source said at the time: ''It lasted about 10 minutes but the girl stayed after and chatted to guests. She took her clothes off but kept her bra and pants on. They wanted to embarrass him.''
Despite insisting they are not dating, Harry and Kendall have been inseparable over the last few months, flying between the UK and the US to see each other and even going on a short ski holiday.
There's already an Oscars buzz surrounding this movie.