The invention relates to a device for welding thermoplastic tubes (1, 2), to a welding method, and to a welding knife. The device according to the invention is provided for welding thermoplastic tubes (1, 2), wherein the tubes (1, 2) are placed into tube holders (100, 200) and squeezed with the aid of tube clamps. A welding knife is moved between the first and the second tube holder and severs the tubes (1, 2) in the process. Cut ends and residual ends are thereby produced. The cut ends are aligned with one another by a relative movement of one tube holder (100) with respect to the other tube holder (200) and, by a horizontal movement of the two tube holders (100, 200), are welded simultaneously to form continuous tubes.
