Viscount Mersey, of Toxteth in the County Palatine of Lancaster, is a title in the Peerage of the United Kingdom. It was created in 1916 for the lawyer and politician John Bigham, 1st Baron Mersey. He had already been created Baron Mersey, of Toxteth in the County Palatine of Lancaster, in 1910, also in the Peerage of the United Kingdom. His son, the second Viscount, was a Deputy Speaker of the House of Lords and also served as Liberal Chief Whip in the House of Lords from 1944 to 1949. His son, the third Viscount, married Katherine Petty-Fitzmaurice, 12th Lady Nairne, the eldest daughter of Henry Petty-Fitzmaurice, 6th Marquess of Lansdowne and 10th Lord Nairne. They were both succeeded by their son, the fourth Viscount Mersey and thirteenth Lord Nairne. As of 2017[update] the titles are held by the latter's son, the fifth Viscount, who succeeded in 2006.
The family seat is Bignor Park, near Pulborough, Sussex.
The heir presumptive (to the Mersey but not Nairne titles) is the present holder's cousin Charles Richard Petty Bigham (b. 1967).