yes, dimmer switch, or any type of variable resistor intended for 120v applications. one question.... why? if you lower the voltage supplied to your pump, you're going to lose flow and possibly overstress the pump.
before tackeling a project like this, you must know that AC power works differently than DC power. if you lower the voltage for the AC device it might not work, or it might just stop working all together. if it does work to lower the voltage, then you will still be stressing the pump way too much.
i dont claim to be an expert on DC or AC electricity, but i have read several posts warning about lowering the voltage to pumps.