Sir Justinian Isham III (20 July 1687 – 5 March 1737), 5th Baronet of Lamport, Northamptonshire was a Member of Parliament during the reign King George II of England.
Justinian Isham III was born on 20 July 1687 to Sir Justinian Isham, 4th Baronet of Lamport, Northamptonshire and his wife Elizabeth Turnor. He became Baronet of Lamport and inherited the family manor house of Lamport Hall on 13 May 1730 when his father died. He was narrowly elected to Parliament over a Whig opponent just a few days later on 21 May 1730, and he represented Northamptonshire for the next four years, voting faithfully with the Tory party. He was returned to Parliament in 1734, when he ran unopposed. He died suddenly on 5 March 1737, probably from a stroke or heart attack, and was succeeded as baronet by his younger brother Edmund Isham, who also followed him as the Northamptonshire representative to Parliament. There is a painting of Sir Justinian by Hugh Howard that hangs at Lamport Hall in Northamptonshire.