Yes. You can return a rental car early, but whether you can get any refund, or even be penalized for the early return, will vary on the terms and conditions stated in your rental agreement.
Car rental companies such as Alamo, Avis, Dollar, Fox, Sixt and Thrifty charge early return fees when you return your car early. On Thrifty's website, it indicates that "early return applies if you did not prepay your rental charges and returned the vehicle more than 24 hours early before the return time indicated in your rental agreement. You will only pay for the days used, but there will be a $25 charge to partially compensate Thrifty for our inability to rent your vehicle during the remaining time reserved for your use. You can reduce the fee paid by calling the On-Rent Help Desk at 1-877-283-0898 to update the date and time of your return.
A few agencies, on the other hand, recalculate the rental charges for early returns. These include Advantage, Budget and Hertz. On Budget's website, it states that "Budget's pricing is based on your confirmed length of rental, as you indicated when you booked your reservation. Our rates vary, depending on whether you rent for one day, one week, or one weekend. So, if you return your car earlier or later than your designated return date, it could affect your price in either direction. Typically, the longer your rental period, the more you save on your per-day rate. Therefore, if your original confirmed rate was for a seven-day rental and you decide to return the car after just three days, your per-day price could increase, which could increase your overall rental fee as well."
Car rental companies like Enterprise, E-Z and National don't penalize early returns. However, there are no refunds for early returns.
Read more about returning your rental car.