The former England star thinks United need a new centre-forward if they are to compete for the title, and Merson believes Kane is the ideal candidate.
He wrote in his Sky Sports column: "I think Manchester United need a centre-forward. I really do. And if they put in a massive bid for Harry Kane, they become massive players next season in the Premier League.
"People say Anthony Martial has done great, and he has done well, and Mason Greenwood will be a superstar, but if you can get Kane playing down the middle, with Martial, Greenwood and Rashford rotating, Kane is getting you 20 goals in a bad season!"
Merson believes that Martial is too inconsistent for United to pin their title hopes on.
He also thinks United need a "natural goalscorer" in their ranks.
Merson - who played for the likes of Arsenal and Aston Villa during his career - explained: "Martial is a hot player; when he is hot, he is brilliant. But you don't win the league on hot and cold. You win the league on consistency, and Harry Kane brings you that, he's the best striker in the Premier League.
"He can score out of nothing when things aren't going well, and United need a natural goalscorer.
"There will also be games where United need someone to hold the ball up. They won't dominate every game with free-flowing football, and sometimes when that ball goes up it has to stick.
"If United buy Harry Kane and a centre-half partner for Harry Maguire, they will be massive players next year."