Pierre Lambert (real name Pierre Boussel; June 9, 1920 – January 16, 2008) was a French Trotskyist leader, who, for many years acted as the central leader of the French Courant Communiste Internationaliste (CCI) which founded the Parti des Travailleurs.
He was born in Paris to a family of Russian Jewish immigrants. Lambert began his activity as a Trotskyist militant before the Second World War when he was a member of the Internationalist Workers Party (POI) led by Raymond Molinier. After the war he continued his activism, as a member of the now united French section of the Fourth International, the Parti Communiste Internationaliste (PCI).
In the PCI he was known as a specialist in trade union matters. When Michel Pablo, the secretary of the Fourth International, raised the question of entrism sui generis he eventually came to oppose this and helped to challenge Pablo within the French Section of the FI, backing the PCI leadership around Marcel Bleibtreu (also known as Pierre Favre).
Differences between Lambert and Bleibtreu forced the latter to leave the PCI. By this time, 1952, the PCI had split into two mutually hostile groups on the question of entrism sui generis and the associated perspective of hundreds of years of deformed workers states propagated by Pablo.
As leader of the PCI by 1954 Lambert forged an alliance with the Socialist Workers Party in the United States and others opposed to Pablo. Lambert lead the PCI to join with these anti-Pabloist fores to form the International Committee of the Fourth International (ICFI). This alliance would last for nearly a decade at which point the SWP (US) and its international faction fused with the International Secretariat of the Fourth International, within which Pablo was now marginalised, to form the United Secretariat of the Fourth International.