What is the possessive form of the name Tess?

The possessive form of the name Tess is "Tess's." This form is used to indicate that something belongs to Tess, such as "Tess's book" or "Tess's car." In some cases, especially in classical usage, it can also be written as "Tess’" without the additional "s," but "Tess's" is generally preferred in modern English.