Spiegeleisen (German: Spiegel— mirror or specular ; Eisen—iron) is a ferromanganese alloy containing approximately 15% manganese and small quantities of carbon and silicon. Historically, this was the standard form in which manganese was traded and used in steel making (see Bessemer process); today, manganese is usually traded and used in more concentrated form, 80% manganese content being typical.

Spiegeleisen is sometimes also referred to as specular pig iron, Spiegel iron, just Spiegel, or Bisalloy.