Hull strawberries. Place in a food processor and process until smooth.
Slowly pour strawberry puree through a fine-mesh sieve and into a 2-quart pitcher. The puree will be thick, but push it through the sieve with a wooden spoon to get as much as possible into the pitcher. Then pour pineapple juice slowly over the puree remaining in the sieve. Continue to stir and press through as you pour the pineapple juice. Discard all leftover strawberry pulp.
Pour strawberry seltzer into the pitcher, stirring with the strawberry puree and pineapple juice until well combined.
Add ice to 6 glasses. Pour punch into glasses. Garnish with mint and fresh strawberries.