Alexander Godfried Gerardus Maria (Xander) Tielens is a Dutch astronomer. He is Adjunct Professor at the University of Maryland, College Park and Emeritus Professor at Leiden Observatory, Leiden University, in the Netherlands. He previously worked at NASA Ames Research Center, California and the University of California, Berkeley, and was previously a professor at the University of Groningen. In 2012, Tielens received the highest distinction in Dutch science, the Spinoza Prize, and he was the most cited astronomer in the Netherlands.