A method and device are provided for determining the transmembrane pressure during an extracorporeal blood treatment in which blood flows at a defined blood flow rate through an arterial blood conduit of an extracorporeal blood circuit into the inlet of a first chamber of a dialyzer, which is divided by a semipermeable membrane into the first chamber and a second chamber, and flows through a venous blood conduit from the outlet of the first chamber of the dialyzer, while dialysis liquid flows through a dialysis liquid supply conduit into the inlet of the second chamber of the dialyzer and flows through a dialaysis liquid discharge conduit from the outlet of the second chamber of the dialyzer. The method and the device for determining the transmembrane pressure are such that the pressure on the blood side and on the dialysis liquid side of the dialyzer is measured with relatively little technical outlay, specifically with fewer than four pressure sensors, and a preliminary uncorrected value is calculated for the transmembrane pressure and is thereafter corrected by a correction variable that is dependent on a variable correlating with the viscosity of the blood.