The constituency is named after the River Itchen, which flows through it and is the lesser of the two major rivers that reach the tidal estuary of Southampton Water at the city. Although until the 1970s it was a safe Labour seat, it elected a Conservative MP, Christopher Chope in 1983 and 1987 after the sitting MP Bob Mitchell left Labour in 1981 for the SDP. The combination of Mitchell as a strong SDP-Liberal Alliance candidate in both 1983 and 1987, together with Conservative landslides, made Southampton Itchen highly competitive.
The former Labour MP, John Denham, narrowly defeated Chope by 551 votes in 1992 and held the seat with more substantial majorities until 2010 when he won by just 192 votes. These two election results with marginal majorities, with moderate third-party polling, and the Conservative gain in 2015, indicate Southampton Itchen may now be thought of as a two-party marginal seat.
Royston Smith unexpectedly regained the seat for the Conservative Party in 2015. He had previously led the Conservative group on the city council and had contested the seat in 2010.