| Viscountcy of Mersey | |
| Creation Date: | 22 January 1916 |
| First Holder: | John Charles Bigham, 1st Viscount Mersey |
| Present Holder: | Edward John Hallam Bigham, 5th Viscount Mersey |
| Heir Presumptive: | Hon. David Edward Hugh Bigham |
| Family Seat: | Bignor Park |
| Motto: | I Advance |
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. 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 uncle Hon. David Edward Hugh Bigham (b. 1938).
The heir presumptive's heir apparent is his son Charles Richard Petty Bigham (b. 1967).
The heir presumptive's heir apparent's heir apparent is his elder son Caspar Patrick Ronan Bigham (b. 1997)