The Castle Museum, previously known as Castle Station or Saginaw Post Office, in Saginaw, Michigan, United States is a historic structure on the National Register of Historic Places.

The Castle Museum was designed by William Martin Aiken, the Supervising Architect of the US Treasury, and built in 1898 as a United States Post Office under a federal building program to reflect the ethnic and cultural heritage of its community.

[1] Aiken used the influence of the European chateau to illustrate the Saginaw Valley's earliest white settlers—French fur traders and trappers—and combine it with decorations from the Italian Renaissance and Gothic periods.

Unable to keep up with Saginaw's growing mail business, the Castle Post Office Station was too small and plans were made to replace it.

Construction was completed in 1937 and Postmaster General James A. Farley labeled the building “memorial to President Roosevelt”.