In a large cup or bowl, combine the pineapple juice, rice vinegar, ketchup, white sugar, and soy sauce. Mix thoroughly.
In a separate small cup, mix the water and cornstarch until smooth to create a slurry.
Pour the pineapple juice mixture into a saucepan over low to medium heat. Whisk continuously until the sugar dissolves.
Add the cornstarch slurry to the saucepan and continue whisking.
Cook until the sauce thickens to the desired consistency.
Stir in the ground black pepper.
Remove from heat and let cool slightly before using.
Store in the refrigerator for up to 3 days.