Frederick's defeats on the battlefield brought still more opportunistic nations into the war. Sweden declared war on Prussia and invaded Pomerania with 17,000 men. Sweden felt this small army was all that was needed to occupy Pomerania and felt the Swedish army would not need to engage with the Prussians because the Prussians were occupied on so many other fronts.

The Pomeranian War was characterized by a back-and-forth movement of the Swedish and Prussian armies, neither of whom would score a decisive victory. It started when Swedish forces advanced into Prussian territory in 1757, but were repelled and blockaded at Stralsund until their relief by a Russian force in 1758. In the course of the following, renewed Swedish incursion into Prussian territory, the small Prussian fleet was destroyed and areas as far south as Neuruppin were occupied, yet the campaign was aborted in late 1759 when the undersupplied Swedish forces succeeded neither in taking the major Prussian fortress of Stettin (now Szczecin) nor in combining with their Russian allies.

A Prussian counter-attack of Swedish Pomerania in January 1760 was repelled, and throughout the year Swedish forces again advanced into Prussian territory as far south as Prenzlau before again withdrawing to Swedish Pomerania in the winter. Another Swedish campaign into Prussia started in the summer of 1761, but was soon aborted due to shortage of supplies and equipment. The final encounters of the war took place in the winter of 1761/62 near Malchin and Neukalen in Mecklenburg, just across the Swedish Pomeranian border, before the parties agreed on the Truce of Ribnitz on 7 April 1762. When on 5 May a Russo-Prussian alliance eliminated Swedish hopes for future Russian assistance, and instead posed the threat of a Russian intervention on the Prussian side, Sweden was forced to make peace.

The war was formally ended on 22 May 1762 by the Peace of Hamburg between Prussia, Mecklenburg and Sweden.