The person to officially rename the Byzantine capital "Istanbul" was the Turkish Sultan Mehmed II, who completed the final siege of the city. "Istanbul" is the way the Ottoman Turks pronounced Constantinople.
Istanbul has had several names. When the Roman Emperor Constantine made Byzantium a new Roman capital, the city's name was changed to Constantinople in the Emperor's honor. According to the history department of the government of Istanbul, Turks and Armenians referred to the city as "Polis." In Greek, the phrase "into the city" translates to "cis tin Polin." The Turks pronounced this as "Istanbul." After the Turks conquered Constantinople under the leadership of Mehmed II, "Istanbul" became the standard pronunciation of the city's name.