Is it Romanoff of Romanov?
In Russian, the family name is Романов which is directly transliterated from Cyrillic into Roman letters as “Romanov”.
Under the empire, however, the standard transliteration format used in diplomacy was French. In the French transliteration, many Russian family names became well-known in the West. “Romanoff” “Galitzine” “Cheremeteff” and “Youssoupoff” became the most common spellings seen outside of Russia. Today, these names are transliterated more literally in English as “Romanov” “Golitsyn” “Sheremetev” and “Iusupov”, but the French transliterations remain very well-known, more familiar, and are still in use.
As the members of the Imperial House settled abroad, they began to use the name of the house as their legal surname under European and American civil laws, and so the members of the family used the spelling “Romanoff”.
Today, most journalists, historians, and others use the more common “Romanov” as does the Russian Federation in its standardized transliterations. In other words, either form is perfectly correct. On this website, they may be used interchangably: when we refer to a current member of the family in a way which requires the use of the surname, we will use the civilly legal spelling “Romanoff” but when we speak of the historical House, or the past members of the family of historical interest, we will use “Romanov”.