This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
Yes, it is. There are three different packages available:
GfxBase offers a commercial version which is said to be excellent, but expensive. The distribution includes different window managers and clients. A demo version is on Aminet. (`gfx/x11/GfxBase-X11-Demo.lha')
AmiWin is a Shareware X11R6 product supporting AmiTCP, AS225 and different graphic cards. A demo version is available on Aminet, `gfx/x11/AmiWin*.lha'.
DaggeX is freely distributable and probably not finished yet. (It calls itself version 0.91.) Source: Aminet, `gfx/x11/DaggeX-0.91.lha' and `gfx/x11/twm_930531.lha'.