John Rennie (August 30, 1794 - September 3, 1874) was the second son of engineer John Rennie and brother of George Rennie. In 1815 he assisted his father in the erection of Southwark Bridge, and in 1819 he went abroad for the purpose of studying the great engineering works on the continent. On the death of his father in 1821, he remained in partnership with his brother George, the civil engineering portion of the business being carried on by him, whereas the mechanical engineering was supervised by George. The most important of his undertakings, from 1824, was the construction of London Bridge, the designs for which had been prepared by his father. The bridge was opened in 1831, when he was knighted. As engineer to the admiralty, a post in which he succeeded his father, he completed various works at Sheerness, Woolwich, Plymouth, Ramsgate, and the great breakwater at Plymouth. Many years of his life were spent in making additions and alterations to various harbors on different parts of the coast, both in England and in Ireland. He retired from the active duties of his profession about 1862, and died in 1874 at the age of 80.