Peter Jansen Wester (23 September 1877–1931; occasionally referred to himself as Peter Johnson Wester) was a Swedish-American agricultural botanist.
Wester was born in Arbra, Haelsingland, Sweden 23 September 1877, he studied biology at Gefleborgs Lans Folkhogskola in Bollnäs and emigrated to the United States in 1897.
Wester worked in several agricultural offices from 1897 to 1903, including leading the United States Department of Agriculture's experiment station and experimental plots for subtropical plants in Miami.
He was a special agent in the USDA Bureau of Plant Industry in Miami and Washington from 1904 to 1910.
He spent the remainder of his career in the Philippines, working with the Philippine Bureau of Agriculture from 1911 until his death in 1932.
While there he was leader of Lamao Experimental Station and an advisor to the Minister.
He made several trips to Singapore, Java, and Hawaii in order to study coffee cultivation and other crops for the purpose of introducing them to the Philippines.Wester's field of interest were spermatophytes, particularly citruses.