I would check what the current requirement is, for the activating coil of the relay.
Using a transistor to activate the relay coil, means that the driving current for the coil does not have to be sourced entirely from the digital output pin of the arduino. But that current still has to come from somewhere. If you are going to provide that current from the +5V of the arduino board, you need to check that it is within the limits of the voltage regulator on the arduino board to supply, and whatever is upstream of that. Otherwise, you may need a separate +5V or similar supply, to provide the current for the relay coil.