The invention provides an apparatus and method for selecting an online advertisement. An apparatus for selecting an online advertisement based on contents sentiment and intention analysis includes a context analysis unit for analyzing a context of contents, a context matching advertisement recommendation unit for selecting an advertisement matching with the context of the contents from an advertisement database (DB) based on the result of the analyzed context, an sentiment information analysis unit for analyzing an sentiment object and sentiment information variously described in the contents based on the result of the analyzed context, an intention recognition unit for recognizing a writing intention of the contents, and an advertisement selection unit for excluding the selected advertisement for the contents or selecting an alternative advertisement depending on the result of the analyzed context, the result of the analyzed sentiment object and sentiment information and the recognized writing intention.