The former West Ham striker congratulated the forward on social media.
The north London side found themselves 2-0 up against West Ham early on - but Carroll duly scored three goals before they struck again.
It finished 3-3 at Upton Park and Harewood is delighted the hat-trick hero has joined a 'special club' in east London.
It was obviously a smart move for Slaven Bilic to play Carroll against Arsenal in particular, exposing their weakness in the air.
But the Hammers target man's restricted playing style means he can only be effective in certain scenarios.
That limits Carroll's effectiveness for West Ham and certainly limits the 27-year-old's chances of playing for England.
At the same time, however, having the option of using an aerial threat as good as Carroll is a huge bonus for West Ham. If he could stay fit all season, they wouldn't need to sign someone like Christian Benteke and could call upon him as an impact sub whenever they needed.
It's a big if, of course, but credit to Carroll for a phenomenal hat-trick against the Gunners.