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On-surface photocatalytic degradation of Methylene Blue was conducted on a paper surface and the process continuously monitored using paper spray ionization mass spectrometry. Firstly, the photocatalyst (TiO2) was immobilized on the paper surface, which was later cut in a triangle shape. A small amount of an aqueous solution of the dye Methylene Blue, used as a prototype target, was then added to the paper and the system exposed to UV-C radiation. Control experiments (photolysis) were also conducted, in which the paper surface contained no TiO2. All the processes were analyzed by paper spray mass spectrometry using lidocaine as internal standard, which was added to the paper just before analyzes. By measuring the intensities of the ions of m/z 284 (cationic Methylene Blue) and m/z 235 (protonated lidocaine) the performance of both processes (on-surface photocatalysis and photolysis) were compared. Photocatalysis showed a remarkable superior performance to degrade Methylene Blue in comparison to photolysis. The absorption of UV-C radiation by TiO2 probably generated reactive species on the paper surface that caused faster dye degradation. In conclusion, paper spray ionization mass spectrometry proved to be a convenient platform where on-surface reactions can be promptly investigated.