Westerhout 40 or W40 (also designated Sharpless 64, Sh2-64, or RCW 174) is a star-forming region in the Milky Way located in the constellation Serpens. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars. The distance to W40 is 436 ± 9 pc (1420 ± 30 light years), making it one of the closest sites of formation of high-mass O-type and B-type stars. The ionizing radiationfrom the massive OB stars has created an H II region, which has an hour-glass morphology.

Dust from the molecular cloud in which W40 formed obscures the nebula, rendering W40 difficult to observe at visible wavelengths of light. Thus, X-ray, infrared, and radio observations have been used to see through the molecular cloud to study the star-formation processes going on within.

W40 appears near to several other star-forming regions in the sky, including an infrared dark cloud designated Serpens South and a young stellar cluster designated the Serpens Main Cluster. Similar distances measured for these three star-forming regions suggests that they are near to each other and part of the same larger-scale collection of clouds known as the Serpens Molecular Cloud.