A big-box store (also hyperstore, supercenter, superstore, or megastore) is a physically large retail establishment, usually part of a chain of stores. The term sometimes also refers, by extension, to the company that operates the store. The term "big-box" references the typical appearance of buildings occupied by such stores.

Commercially, big-box stores can be broken down into two categories: general merchandise (examples include Walmart, Target, and Kmart), and specialty stores (such as The Home Depot, Barnes & Noble, or Best Buy), which specialize in goods within a specific range, such as hardware, books, or consumer electronics, respectively. In the late 20th and early 21st centuries, many traditional retailers and supermarket chains that typically operate in smaller buildings, such as Tesco and Praktiker, opened stores in the big-box-store format in an effort to compete with big-box chains, which are expanding internationally as their home markets reach maturity.

The store may sell general dry goods, in which case it is a general merchandise retailer (however, traditional department stores, as the predecessor format, are generally not classified as "big box"), or may be limited to a particular specialty (such establishments are often called "category killers"), or may also sell groceries, in which case some countries (mostly in Europe) use the term hypermarket. In the U.S., there is no specific term for general merchandisers who also sell groceries. Both Target and Walmart offer groceries in most branches in the U.S.

Big-box stores are often clustered in shopping centers, which are typically called retail parks in the United Kingdom. In the United States, when they range in size from 250,000 square feet (23,000 m²) to 600,000 square feet (56,000 m²), they are often referred to as power centers.