Hi, what is the current required to drive the coil of your relay, if its more than 20mA then you will need a transistor to do the switching.
I personally would never drive a coil directly from an output.
No matter what you have, if its a semiconductor activating an inductive like a coil, you will need a diode.
Cathode to positive side of coil and Anode to negative side of coil, this will protect your arduino output from inductive spikes.
Tom..... 