An online marketplace (or online e-commerce marketplace) is a type of e-commerce website where product or service information is provided by multiple third parties. Online marketplaces are the primary type of multichannel ecommerce and can be a way to streamline the production process.

In an online marketplace, consumer transactions are processed by the marketplace operator and then delivered and fulfilled by the participating retailers or wholesalers. These types of websites allow users to register and sell single items to many items for a "post-selling" fee.

Because marketplaces aggregate products from a wide array of providers, the selection is wider, and availability is higher than in vendor-specific online retail stores. Some online marketplaces have a wide variety of general interest products that cater to almost all the needs of the consumers, others are consumer specific and cater to a particular segment. Online marketplaces became abundant in 2014.

Business-to-business (B2B) online marketplaces are platforms that allow companies to buy and sell products or services to other businesses. These marketplaces typically focus on a specific product or service category and are used by businesses to find suppliers, negotiate prices, and manage logistics.

Some examples of B2B online marketplaces include VerticalNet, Commerce One, and Covisint, which were some of the earliest B2B marketplaces to emerge in the early days of e-commerce. More contemporary B2B marketplaces include EC21, Elance, and eBay Business, which focus on specific product or service categories and facilitate complex transactions such as requests for quotations (RFQs), requests for information (RFIs), and requests for proposals (RFPs).

Online marketplaces are information technology companies that act as intermediaries by connecting buyers and sellers. Examples of prevalent online marketplaces for retailing consumer goods and services are Amazon, Taobao and eBay. On the website of the online marketplace sellers can publish their product offering with a price and information about the product's features and qualities. Marketplace sellers often utilize a marketplace integrator or channel integration software to efficiently list and sell products across multiple online marketplaces. Potential customers can search and browse goods, compare price and quality, and then purchase the goods directly from the seller. The inventory is held by the sellers, not the company running the online marketplace. Online marketplaces are characterized by a low setup cost for sellers, because they do not have to run a retail store. While in the past Amazon Marketplace has served as a role model for online marketplaces, the expansion of the Alibaba Group into related business such as logistics, e-commerce payment systems and mobile commerce is now trailed by other marketplace operators such as Flipkart.

For consumers, online marketplaces reduce the search cost, but insufficient information on the quality of goods and an overloaded goods offering can make it more difficult for consumers to make purchasing decisions. Consumers' ability to make a purchasing decision is also hampered by the fact that an online marketplace only allows them to examine the quality of a product based on its description, a picture and customer reviews. Another characteristic of online marketplaces is that the same product can be offered by several merchants. In this case, consumers can often make the selection of a merchant with the support of reviews of that merchant, for example. Despite many conceivable factors influencing merchant selection, such as convenience, seller ratings, delivery options and a wider selection of goods, customers choose primarily on the basis of the lowest price for a particular product.

Peer-to-peer (P2P) online marketplaces enable direct transactions between individuals, often facilitated by an intermediary platform that provides services such as payment processing, dispute resolution, and user verification. Unlike traditional e-commerce platforms that primarily follow a business-to-consumer (B2C) model, P2P marketplaces allow users to act as both buyers and sellers, fostering decentralized commerce