For this knowledge, the discovery of the chromosome trisomy 22 in siamang (Symphalangus syndactylus) is the first report. To our scheme, blood sample was collected from one female of Khao Kheow Open Zoo (KKOZ), Chonburi province, Thailand. Chromosomes were prepared by lymphocyte cells culture at 37°C for 72 h then stained by G banding technique. The result indicated that diploid (2n) of siamang was 51 (one irregular chromosome was found). The autosome consists of 47 metacentric and submetacentric, 2 telocentric chromosomes. The X chromosome was metacentric chromosome. The increasing of one metacentric chromosome 22 lead to the presence of the irregular chromosome and the changing of 2n=50 to 2n=51. The augmentation of chromosome 22 was found in all metaphase cells of the entire study. We suggested that the chromosome increasing is a consequence from the non-disjunction in meiosis cell division.