Here we present a small-scale differential refractive index sensor configuration based on localized surface plasmon resonance (LSPR) sensing on top of a photodiode array. A thin gold film containing short-range ordered holes with localized SPR properties is placed onto two photodiode elements. The LSPR substrate is covered by two immersion media having one known (reference) on the first photodetector element and one unknown refractive index on the second. Transmitted light is detected and a lateral position of maximum light transmission (Pos_MLT) between the two photodetector elements is calculated. Multiple incident wavelengths are chosen to compute the lateral Pos_MLT in the wavelength regime of 620-640 nm, which coincides with a region of linear LSPR extinction. The sensor layout is tested with media of varying refractive indices. A linear dependency between the change of Pos _MLT per wavelength shift on the change of the refractive index is obtained and determined to be of 203.2 μm/nm per refractive index unit (RIU) for our LSPR substrate. The obtained results are explained by the extinction properties of the LSPR substrate. The resolution limit is governed by the statistical noise if the difference in refractive index decreases, which is discussed here in detail. With this small-scale sensor configuration, the determination of refractive index changes can be carried out with light incident normal to the plane of the sensor. Furthermore, the evaluation of the transmitted light can be conducted without the use of spectrometers and bulky optics. The sensor principle outlined here can be adapted to other LSPR substrates and can be employed for various sensing tasks, e.g. in water monitoring, chemical industries or medical/biological lab on a chip applications.