The 26-year-old rapper wants the One Direction star to stay away from his girlfriend, Ariana Grande, and hates that they have stayed in touch since working on the song 'Just A Little Bit of Your Heart' together.
A source told America's OK! magazine: ''Big Sean doesn't like it that Harry's been wooing her. He wants Harry to back off and leave Ariana alone.''
The British singer, 20, penned the track especially for the 21-year-old beauty earlier this year.
Big Sean recently insisted he and Ariana have ''great chemistry.''
He said: ''Me and her make a lot of music together and the chemistry between me and her when we work is great.
''We got more in the works too. I feel like her voice is crazy, [just] that that big voice comes out of her is insane. I was probably one of the first people to tweet about her music; me and her been friends for years before anything else...
''I was the first rapper to perform at the White House, I performed there with Ariana so that was an honour.''
The couple's romance, which began two months ago, came as a shock to fans as Big Sean - whose real name is Sean Anderson - only ended his five-month engagement to 'Glee' actress Naya Rivera in April this year.