Is Jarrett a first name or a last name?

Jarrett is both: a family name in some places and a given name in others. It is mostly a family name in the United Kingdom.

How to address someone called Jarrett

United States. Jarrett is used both ways: about 21,821 people had it as a surname in the 2010 census, and 21,718 babies born between 1950 and 2017 were given it as a first name. Look at where it sits in the name and how the person signs.
United Kingdom. Jarrett is a family name, usually written last. Write Dear Mr. Jarrett or Dear Ms. Jarrett.

Not sure which part of a full name is which? Check the full name with the country where the person is based.

People recorded in Wikidata who carry Jarrett as a family name or as a given name, by country of citizenship (120 in total).

Country	As family name	As given name	Family name share
United States	46	38	55%
United Kingdom	8	2	80%
Canada	7	2	78%
Australia	6	1	86%

Wikidata covers notable people, so counts show how the name is used, not how many people carry it.

As a surname: about 21,821 people in the 2010 census, rank 1,648 among US surnames.
As a first name: 21,718 babies born between 1950 and 2017, 0% of them girls.