The Orthodox Union is suing a Pennsylvania-based seafood-importing company, saying it falsely claimed the OU certified its tilapia fish as kosher, highlighting a common problem in the increasingly global kosher marketplace.

In a suit filed in federal court in Manhattan this month, the Orthodox Union, a Manhattan not-for-profit with a kosher division, sued Western Edge Inc. for trademark infringement and deceptive trade practice, alleging the unauthorized use of its registered certification mark. The suit alleges that Western...