The Leander-class, or Type 12I (Improved) frigates, comprising twenty-six vessels, was among the most numerous and long-lived classes of frigate in the Royal Navy's modern history. The class was built in three batches between 1959 and 1973. It had an unusually high public profile, due to the popular BBC television drama series Warship. The Leander silhouette became synonymous with the Royal Navy through the 1960s until the 1980s.

The Leander design or derivatives of it were built for other navies:

Royal New Zealand Navy as the Leander class Chilean Navy: Condell class Royal Australian Navy: River class Indian Navy: Nilgiri class Royal Netherlands Navy: Van Speijk class