Wash and dry the shiso leaves. Stack on top of each other, roll up like a cigar, and slice thinly.

Combine sugar and water in a small saucepan and bring to a boil, stirring to dissolve the sugar. Remove from the heat. Add the shiso leaves and let infuse for 4 hours. Strain the syrup into a glass bottle and store, covered, in the refrigerator for up to 2 weeks.