The flag of Genoa is a St. George's Cross, a red cross on a white field.
The patron saint of Genoa was Saint Lawrence until at least 958, but the Genoese transferred their allegiance to Saint George (and Saint John the Baptist) at some point during the 11th or 12th century, most likely with the rising popularity of the military saint during the Crusades. Genoa also had a banner displaying a cross since at latest 1218, possibly as early as 1113. But the cross banner was not associated with the saint; indeed, the saint had his own flag, the vexillum beati Georgii (first mentioned 1198), a red flag showing George and the dragon. A depiction of this flag is shown in the Genoese annals under the year 1227. The Genoese flag with the red cross was used alongside this "Saint George's flag", from at least 1218, known as the insignia cruxata comunis Janue ("cross ensign of the commune of Genoa").
The saint's flag was the city's main war flag, but the cross flag was used alongside it in the 1240s.
The Saint George's flag (i.e. the flag depicting the saint) remained the main flag of Genoa at least until the 1280s. The flag now known as the "St. George's Cross" seems to have replaced it as Genoa's main flag at some point during the 14th century. The Book of Knowledge of All Kingdoms (c. 1385) shows it, inscribed with the word iustiçia, and described as:
And the lord of this place has as his ensign a white pennant with a red cross. At the top it is inscribed with 'justice', in this manner.
There is some debate as to whether the flag of England derives from the flag of Genoa. While some historians point to a lack of evidence, others say that the English adopted the flag on their ships in 1190 in order to benefit from the protection of the Genoese fleet.