We report on a new sensitive method, for bovine serum albumin quantification, which is based on the use of methylene blue as a labelling agent combined with photothermal lens detection. In the presence of sodium dodecyl sulfate, methylene blue forms a dimer which can react with bovine serum albumin producing dedimerization. We found that, the photothermal signal decreases proportional to the concentration of bovine serum albumin added to the system composed by dodecyl sulfate and methylene blue. Therefore, the change of the signal intensity is linearly proportional to the concentration of bovine serum albumin. Under the optimized analytical conditions, used in this work, the photothermal signal is linearly dependent on the concentration of bovine serum albumin in the range of 0.5 × 10-6-7.5 × 10-5 gmL-1 with a regression coefficient R2 = 0.9954. The relative standard deviation for BSA determination at 3.5 × 10-5 gmL-1 (n = 5) is 2.3% and the achieved detection limit is 3.5 × 10-7 gmL-1.