The $60m (£35.7m) modernisation project enables the production of lighter-weight board grades, along with greater finishing flexibility.
Marek Krzykowski, mill general director at Kwidzyn, said: "Operations are fully restored and our new capabilities will allow us to provide even better products and services for our customers."
The investment and International Paper's resulting enlarged product portfolio respond directly to customers' growing demand for economically attractive and environmentally sustainable packaging solutions.
"Our renowned Alaska and Arktika grades continue to be available with unchanged technical specifications following the rebuild", said Michael Krueger, International Paper's commercial director for coated paperboard & recycled specialties. "In addition, we intend to introduce our new differentiated lighter-weight GC2 products by the end of this quarter, and will be communicating further details in the coming week."