The former Republic of Ireland Under-21 international, 24, has played 54 Championship games for his parent club.
Hoban will go straight into the Dons squad for Thursday's Europa League first-leg tie with Burnley, pending the receipt of his international clearance.
Watford say they hold an option to recall him in January.
"Tommie is a player we had last week and thankfully we managed to get the deal resurrected," McInnes told RedTV.
"First and foremost he's an excellent defender. It's well known that we've been in the market for another defender and to get a player of his quality from Watford is a great piece of business.
"Although he can play anywhere in the defence, centre-back is his preferred position."