Optimisasi mesin pencari (bahasa Inggris: Search Engine Optimization, biasa disingkat SEO) adalah serangkaian proses yang dilakukan secara sistematis yang bertujuan untuk meningkatkan volume dan kualitas trafik kunjungan melalui mesin pencari menuju situs web tertentu dengan memanfaatkan mekanisme kerja atau algoritma mesin pencari tersebut.

Tujuan dari SEO adalah menempatkan sebuah situs web pada posisi teratas, atau setidaknya halaman pertama hasil pencarian berdasarkan kata kunci tertentu yang ditargetkan. Secara logis, situs web yang menempati posisi teratas pada hasil pencarian memiliki peluang lebih besar untuk mendapatkan pengunjung.

Sejalan dengan makin berkembangnya pemanfaatan jaringan internet sebagai media bisnis, kebutuhan atas SEO juga semakin meningkat. Berada pada posisi teratas hasil pencarian akan meningkatkan peluang sebuah perusahaan pemasaran berbasis web untuk mendapatkan pelanggan baru. Peluang ini dimanfaatkan sejumlah pihak untuk menawarkan jasa optimisasi mesin pencari bagi perusahaan-perusahaan yang memiliki basis usaha di internet.

Search engine optimization (SEO) is the process of improving the volume or quality of traffic to a web site from search engines via "natural" or un-paid ("organic" or "algorithmic") search results as opposed to search engine marketing (SEM) which deals with paid inclusion. Typically, the earlier (or higher) a site appears in the search results list, the more visitors it will receive from the search engine. SEO may target different kinds of search, including image search, local search, video search and industry-specific vertical search engines. This gives a web site web presence.

As an Internet marketing strategy, SEO considers how search engines work and what people search for. Optimizing a website primarily involves editing its content and HTML and associated coding to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines.

The acronym "SEO" can also refer to "search engine optimizers," a term adopted by an industry of consultants who carry out optimization projects on behalf of clients, and by employees who perform SEO services in-house. Search engine optimizers may offer SEO as a stand-alone service or as a part of a broader marketing campaign. Because effective SEO may require changes to the HTML source code of a site, SEO tactics may be incorporated into web site development and design. The term "search engine friendly" may be used to describe web site designs, menus, content management systems, images, videos, shopping carts, and other elements that have been optimized for the purpose of search engine exposure.

Another class of techniques, known as black hat SEO or spamdexing, use methods such as link farms, keyword stuffing and article spinning that degrade both the relevance of search results and the user-experience of search engines. Search engines look for sites that employ these techniques in order to remove them from their indices.