The 27-year-old German striker has signed for the Gunners this summer already and will join an attacking line-up that includes of Robin van Persie, Gervinho, Theo Walcott and Marouane Chamakh.
The writer of Fever Pitch, High Fidelity and About A Boy believes the former Bayern Munich forward can be a success at the Emirates Stadium.
He told German publication Kolner Stadt Anzeiger: "He can become the biggest Arsenal star, the biggest star in our capital. And our hero.
"I have watched all his goals for Cologne that he scored this season.
"When it became more probable that he would sign for Arsenal, I watched him closer. I'm fully convinced by him."
Hornby also predicted Germany will triumph at Euro 2012 over favourites Spain and group competitors Netherlands, Denmark and Portugal.
He said: "It's an opportunity: We'll get drunk. Germany's going to win the tournament."