There's one catch, however: you can't select just any drink from the menu for your refill. Your options only include Starbucks' lower-priced drinks, like brewed coffee, iced coffee, cold-brew coffee, hot tea, and iced tea.
To take advantage of the policy, you have to pay for your original drink using a Starbucks card or the coffee chain's mobile app.
Once you finish the drink, you can ask a barista for a free refill of your choice.
RELATED: Never order these items at Starbucks
10 worst Starbucks deals
10 worst Starbucks deals
1. Coffee Beans and Grounds
If you're looking to brew some Starbucks coffee on your own, skip out on heading to the brick-and-mortar store to buy it.
"Starbucks sells 16-ounce packages of whole bean and ground coffee for $11.95 to $14.95, depending on the blend," said Perez. "However, 12-ounce bags of the same blends can be fetched for as little as $7.95 at Target, while Albertson's has been known to offer a sale price of just $6.99. [The] bottom line: Do not buy your bagged Starbucks coffee from Starbucks."
Also, not only is the in-store coffee priced higher than you would find at other retailers, but if you shop on the Starbucks store online, you can often find coupons that can take off as much as 20 percent off your entire order, said Sherin.
Like Starbucks coffee beans and grounds, Starbucks K-Cups are generally not worth the purchase. K-Cups are the small plastic pods your Keurig might use to make a serving of coffee. And, although brewing coffee at home is usually an easy way to save money, it's not with K-Cups.
"Never buy these from an actual Starbucks location," said James. "You can always find them much cheaper at Costco."
3. Travel Mugs and Drinkware
Starbucks-brand drinkware is another "deal" that isn't a smart choice for your wallet. "Impressive displays of coffee mugs, tea kettles, French presses and other coffee and tea accessories are tempting for those waiting in line for drinks, but are best skipped," said Perez.
"Most coffee mugs start at $9.95 and cost upwards of $29.95," she added. "The Limited-Edition travel mugs are an especially bad deal — priced between $70 and $74.95, these mugs further Starbucks' obsession with Swarovski crystals and make absolutely no sense at all."
If you want to buy Starbucks drinkware, approach it like buying Starbucks coffee beans for home, said Sherin. "Shop other online retailers like Amazon, or find a coupon for the Starbucks store online to save even more," she said.
4. Starbucks Gift Cards
Starbucks gift cards stare you in the face every time you reach the cashier. Although they are a great gift to receive, you can save on them by purchasing elsewhere.
"Buying gift cards can be a big mistake if you purchase through a retailer," said Sherin. "Use sites like Raise.com or Cardpool.com and you can find discounts as much as 10 percent off."
5. Premium Gift Cards
Speaking of gift cards, Starbucks offers special ones around the holidays — but they're more a money trap than a deal, said Mark Mazza, co-founder of PromoCodeWatch.com.
For instance, Perez said, "Every holiday season, Starbucks comes out with a high-priced gift card which costs $200 but only has $50 preloaded. Trust me, no one on your shopping list wants you to pay $200 for a $50 value, even if the card is made of stainless steel or encrusted with Swarovski crystals.
6. Bottled Water
Bottled water is definitely not a Starbucks deal. There's no reason to spend up to $5 on Ethos or Fiji water when you can get triple-filtered water for free, said Perez.
7. Bistro Boxes
Another Starbucks item to be wary of are its lunch bistro boxes.
“Five dollars for a few crackers, cheese cubes, grapes and a hard-boiled egg is not a good deal, given how inexpensively these items are at your local grocery store,” said Perez. “Starbucks' latest offering of lunch, two snacks and a bottled water for $8, is a bit easier to swallow...still, you can pack all these things from home and save your money for a fancy latte."
8. Prewrapped Sandwiches
Along with Starbucks lunch boxes, the cafe's expensive pre-wrapped sandwiches are another deal to avoid.
“They display these nicely wrapped sandwiches that look nice and have quality ingredients as you wait in line to hook you,” said Mazza. “Coffee is relatively cheap, so selling you an overpriced sandwich that costs as much, or more, than your drink order is an obvious score for them."
9. Breakfast Sandwiches
Continuing on the sandwich theme, Starbucks breakfast sandwiches should probably not be on your list of things to buy. They look fresh in the display, but Starbucks breakfast sandwiches, as well as many other food items, are simply overpriced for the quality.
10. Blended Crème Frappuccino
Unless you’re a child or watching your caffeine intake, blended crème Frappuccinos don’t make sense for your money. "It's basically a $5 vanilla milkshake, and not a very good one at that. Way too much ice in this thing and not enough thickness," said James. Blended crème Frappuccinos will cost you in calories as well, considering a 16 oz. vanilla bean grande frappuccino packs 400 calories.
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Since the policy doesn't apply to customers who leave the store after purchasing their original drink, free refills aren't available to customers via drive-thru.
Even if you follow the rules, however, be prepared to run into some baristas who aren't familiar with the policy.
Seattle attorney Melody Overton, who runs the blog StarbucksMelody.com, has called the refills one of Starbucks' biggest "problem policies" because some baristas aren't aware of it.