Four species of Mastrus Förster, 1869 are reported from China. Two, Mastrus nigrus Sheng & Zeng, sp. n. reared from Arge pullata (Zaddach) and Mastrus rugotergalis Sheng & Zeng, sp. n. reared from Diprion jingyuanensis Xiao & Zhang, are new to science. One, Mastrus deminuens (Hartig, 1838), is a parasitoid of Pachynematus itoi Okutani. A key to species of Mastrus Förster known in China is provided.