Siyi (Seiyap or Sze Yup in Cantonese; Chinese: 四邑方言; pinyin: Sìyì fāngyán; Jyutping: sei3 jap1 fong1 jin4 meaning "Four Hamlets") is a coastal branch of Yue Chinese spoken mainly in Guangdong province, but is also used in overseas Chinese communities.

Within the province, it is mainly spoken in the prefecture-level city of Jiangmen, but pockets exist outside Jiangmen, including the Doumen and Jinwan districts in Zhuhai, Guzhen in Zhongshan and Jun'an in Foshan.

The name "Sze Yup" or "Seiyap" (Chinese: 四邑; pinyin: Sìyì; Cantonese Yale: sei yāp; lit.

Since a fifth county, Heshan, was added to the prefecture in 1983, this region is referred to as the "Five Counties" (Chinese: 五邑; pinyin: Wǔyì; Cantonese Yale: nǵh yāp) in the province; but for historical reasons, the term "Seiyap" is still used amongst the overseas Chinese communities.

Most of the region in which Siyi is spoken is administered by the prefecture-level city of Jiangmen, including the Jiangmen city districts of Jianghai, Pengjiang and Xinhui, as well as the county-level cities of Taishan, Kaiping, Enping and the southeastern part of Heshan, but the dialect is also spoken in parts of Zhuhai, the town of Guzhen in Zhongshan and the town of Jun'an in Foshan.