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 The Rochester Bridge Act 1846, granting the Wardens and Assistants of Rochester Bridge the powers to replace the bridge, received Royal Assent on 14 May 1846. The placement of the new bridge on the line of Watling Street and the old Roman bridge required the purchase of considerable property in Strood before construction could begin. The first contract for the bridge foundations was finally signed in January 1850, and six years later the construction was completed. On 13 August 1856 the Wardens and Assistants, accompanied by a Royal Marine band and the mayors and aldermen of Rochester and Maidstone, processed across the old bridge, along the Strood Esplanade, and back to the centre of the new bridge, where the Victorian cast iron bridge was officially declared open to the public and the medieval stone bridge was declared closed. The momentous day concluded with a public dinner and a fireworks display from the old bridge in the evening. Even more spectacular fireworks from the old bridge followed over the next six months as the Royal Engineers practiced their demolition skills by blowing up the stone structure which, though much repaired and rebuilt, had crossed the River Medway for 465 years.
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The modernisation of the medieval stone bridge proved to be only a temporary solution to the increasing demands of road and river traffic. During the 1840s the Wardens and Assistants of Rochester Bridge considered proposals designed by Bridge Engineer Sir William Cubitt for a new stone bridge of five arches, a suspension bridge, or a cast iron bridge of three arches. At the insistence of the Admiralty they finally decided on a cast iron bridge with three arches and a ship's passage with a swing bridge that would allow ships with fixed masts to navigate upriver. Although delicate in appearance, the massive cast iron structure weighed over 2500 tonnes and rested on foundations of cast iron cylinders sunk through the riverbed onto the bedrock beneath. 
The new bridge was 40 feet wide with a combined span of 485 feet over the three arches and 18 feet of clear headroom above high water. A feature of this bridge was a ship's passage at the Strood end of the bridge with a swing bridge opening to a width of 50 feet to allow large vessels to pass through the bridge. A marvel of Victorian engineering, the swing bridge was so delicately balanced that, even though the total weight of the swing bridge and roadway was over 300 tonnes, two men could rotate it with ease 90 degrees upriver in five minutes.