Shìjīn (市斤) is a traditional Chinese unit of mass used in markets, especially in mainland China and Taiwan. It literally combines 市 (shì), meaning 'market' or 'commercial', and 斤 (jīn), an ancient weight unit. Unlike the imperial catty (≈604.8 g) used historically in Hong Kong and overseas Chinese communities, the modern shìjīn is standardized at exactly 500 grams — half a kilogram — making it convenient for everyday shopping and food sales.

This unit remains widely used in fresh markets, street stalls, and small grocery stores, particularly when weighing fruits, vegetables, meat, and seafood. While official documents and scientific contexts use metric units like grams or kilograms, shìjīn endures in oral communication and informal pricing. Its persistence reflects cultural continuity and practicality: prices are often quoted per shìjīn (e.g., ¥12/shìjīn), and vendors commonly use balance scales calibrated in this unit.