西部马脑炎病毒（英文：Western Equine Encephalitis，简称:WEE）是经由蚊子所感染的病毒，主要感染马类，但也可以感染人。西部马脑炎病毒和东部马脑炎病毒一样都属脑炎病毒的一种。第一次发现于1930年，首次感染人类的案例为1938年病致死。西部马脑炎病毒大都分部于密西西比河以西。但在南美洲等国家也有案例。西部马脑炎病毒相较东部马脑炎病毒更温和，致死程度大概在4%左右。

The Western equine encephalomyelitis virus is the causative agent of relatively uncommon viral disease Western equine encephalomyelitis (WEE). An Alphavirus of the family Togaviridae, the WEE virus is an arbovirus (arthropod-borne virus) transmitted by mosquitoes of the genera Culex and Culiseta. WEE is a recombinant virus between two other alphaviruses, an ancestral Sindbis virus-like virus, and an ancestral Eastern equine encephalitis virus-like virus. There have been under 700 confirmed cases in the U.S. since 1964.