The invention relates to a method 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 6 of an extracorporeal blood circuit 5A into the inlet of a first chamber 3 of a dialyzer 1, which is divided by a semipermeable membrane 2 into the first chamber and a second chamber 4, and flows through a venous blood conduit 7 from the outlet of the first chamber 3 of the dialyzer 1, while dialysis liquid flows through a dialysis liquid supply conduit 10 into the inlet of the second chamber of the dialyzer and flows through a dialaysis liquid discharge conduit 11 from the outlet of the second chamber of the dialyzer. The method according to the invention and the device according to the invention 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 20, 21, 22, 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.