The key to understand the nature of dark energy relies in our ability to probe the distant Universe. In this framework, the recent detection of the kinematic Sunyaev-Zel'dovich (kSZ) effect signature in the cosmic microwave background obtained with the South Pole Telescope (SPT) is extremely useful since this observable is sensitive to the high-redshift diffuse plasma. We analyse a set of cosmological hydrodynamical simulation with 4 different realisations of a Hu & Sawicki f (R) gravity model, parametrised by the values of f R,0 = (0, −10 −6 , −10 −5 , −10 −4 ), to compute the properties of the kSZ effect due to the ionized Universe and how they depend on f R,0 and on the redshift of reionization, z re . In the standard General Relativity limit ( f R,0 =0) we obtain an amplitude of the kSZ power spectrum of D kSZ 3000 = 4.1 µK 2 (z re =8.8), close to the +1σ limit of the D kSZ 3000 = (2.9 ± 1.3) µK 2 measurement by SPT. This corresponds to an upper limit on the kSZ contribute from patchy reionization of D kSZ,patchy 3000 < 0.9 µK 2 (95 per cent confidence level). Modified gravity boosts the kSZ signal by about 3, 12 and 50 per cent for f R,0 = (−10 −6 , −10 −5 , −10 −4 ), respectively, with almost no dependence on the angular scale. This means that with modified gravity the limits on patchy reionization shrink significantly: for f R,0 = −10 −5 we obtain D kSZ,patchy 3000 < 0.4 µK 2 . Finally, we provide an analytical formula for the scaling of the kSZ power spectrum with z re and f R,0 at different multipoles: at ℓ = 3000 we obtain D kSZ 3000 ∝ z re 0.24 1 + f R,0
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