The massacres in Piaśnica were a series of mass murders carried out by Nazi Germany during World War II, between the fall of 1939 and spring of 1940 in Piaśnica Wielka in the Darzlubska Wilderness near Wejherowo. The exact number of people murdered is unknown, but estimates range between 12,000 and 14,000 victims. Most of them were Polish intellectuals from Gdańsk Pomerania, but Poles, Kashubians, Jews, Czechs and German inmates from mental hospitals from the General Government and the Third Reich were also murdered. After the Stutthof concentration camp, Piaśnica was the largest site of killings of Polish civilians in Pomerania by the Germans, and for this reason, is sometimes referred to as the "second" or "Pomeranian" Katyn. It was the first large-scale Nazi atrocity in occupied Poland.

Reichsgau Danzig-West Prussia was an administrative division of Nazi Germany created on 8 October 1939 from annexed territory of the Free City of Danzig, the Greater Pomeranian Voivodship, and the Regierungsbezirk West Prussia of Gau East Prussia.