The wife of a marquess is a marchioness, of an earl a countess, of a viscount a viscountess.
From the telegraph.co.uk
The viscountess was then arrested and taken to Harlow police station, where a blood sample was taken.
From the thisismoney.co.uk
The female equivalent of a viscount is a viscountess.
From the en.wikipedia.org
In the past three weeks, the authoritative London Times has reported the deaths of a countess, a viscountess, a baroness, two lords, two baronets, a knight and the widows of eight knights.
From the time.com
More examples
A wife or widow of a viscount
A noblewoman holding the rank of viscount in her own right
A viscount (-count) is a member of the European nobility whose comital title ranks usually, as in the British peerage, above a baron, below an earl (in Britain) or a count (the earl's continental equivalent).
The wife of a viscount; A female member of the peerage who bears the title in her own right
A lady who has been Princess of a Principality once.