Taiwanese Mandarin or Guoyu (traditional Chinese: 國語; simplified Chinese: 国语; pinyin: Guóyǔ; lit. 'National Language') refers to any of the varieties of Mandarin Chinese spoken in Taiwan. This comprises two main forms: Standard Guoyu and Taiwan Guoyu.

Standard Guoyu (標準國語) refers to the formal variety that serves as the official national language of the Republic of China (Taiwan). This variety is used in the education system, in official communications, and in most news media. The core of this standard variety is described in the dictionary Guoyu Cidian (國語辭典), which is maintained by the Ministry of Education of Taiwan. This dictionary is based on the phonology of the Beijing dialect and the grammar of vernacular Chinese. Standard Guoyu closely resembles, and is mutually intelligible with the Standard Mandarin (普通話; 普通话; Pǔtōnghuà) of mainland China. However, some divergences and differences exist in pronunciation, vocabulary, and grammar.

Taiwan Guoyu (台灣國語) refers to the colloquial form of the language. This comprises varieties of Mandarin used in Taiwan that diverge from Standard Guoyu. These divergences are often the result of Taiwan Guoyu incorporating influences from other languages used in Taiwan - primarily Taiwanese Hokkien and Japanese, to a lesser extent. Like Standard Guoyu, Taiwan Guoyu is mutually intelligible with Putonghua. However, when compared with Standard Guoyu, Taiwan Guoyu exhibits greater differences from Putonghua, and is more identifiably "Taiwanese" as a result.

All forms of written Chinese in Taiwan use traditional characters alongside other Sinophone areas such as Hong Kong, Macau, and many overseas Chinese communities. This is in contrast to mainland China, where simplified Chinese characters were adopted beginning in the 1950s.