Pu'er, Pu-erh, Puer, also Po Lei or Bolay is a variety of post-fermented tea produced in Yunnan province, China. Post-fermentation is a tea production style in which the tea leaves undergo a microbial fermentation process after they are dried and rolled. This is a Chinese specialty and is sometimes referred to as dark tea. There are a few different provinces, each with a few regions, producing dark teas of different varieties. Those produced in Yunnan are generally named Pu'er, referring to the name of Pu'er county which used to be a trading post for dark tea during imperial China.

Pu'er is available as loose leaf or in various compressed forms (see Tea brick). There is also the differentiation of ripened (shou) and raw (sheng) types. The shou type refers to those varieties that have gone through a proper post-fermentation process, while the sheng types are those in the process of gradual darkening through exposure to the environmental elements. Certain selections from either type can be stored for maturity before consumption. That is why some are labelled with year and region of production.

Darkening tea leaves to trade with ethnic groups at the borders has a long history in China. These crude teas were of various origins and were meant to be low cost. Darkened tea is still the major beverage for the ethnic groups in the southwestern borders and, till the early 1990s, was the third major tea category produced by China mainly for this market segment.

There had been no standardized processing for the darkening of pu'er tea until the postwar years in the 1950s, where there was a sudden surge in demand in Hong Kong, because of the concentration of refugees from the mainland. In the 1970s, the improved process was taken back to Yunnan for further development, which has resulted in the various production styles today.