The wife of a marquess is a marchioness, of an earl a countess, of a viscount a viscountess.
From the telegraph.co.uk
Here, according to the legend, he fell in love with Vittoria Colonna, marchioness of Pescara and a poet.
From the en.wikipedia.org
Vittoria Colonna was the marchioness of Pescara.
From the en.wikipedia.org
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The wife or widow of a marquis
A noblewoman ranking below a duchess and above a countess
A marquess or marquis (from French "marquis") is a nobleman of hereditary rank in various European monarchies and some of their colonies. The term is also used to render equivalent oriental styles as in imperial China, Japan, and Vietnam (Annam). ...