We derive constraints on millicharged dark matter and axion-like particles using pulsar timing and fast radio burst observations. For dark matter particles of charge e, the constraint from time of arrival (TOA) of waves is /m milli 10 −8 eV −1 , for masses m milli 10 −6 eV. For axionlike particles, the polarization of the signals from pulsars yields a bound in the axial coupling g/ma 10 −13 GeV −1 /(10 −22 eV), for ma 10 −19 eV. Both bounds scale as (ρ/ρ dm ) 1/2 for fractions of the total dark matter energy density ρ dm . We make a precise study of these bounds using TOA from several pulsars, FRB 121102 and polarization measurements of PSR J0437−4715. Our results rule out a new region of the parameter space for these dark matter models.Unraveling the nature of dark matter (DMa) is among the most urgent issues in fundamental physics. Indirect searches aim at detecting the effects of DMa in astrophysical observations, beyond its pure gravitational interaction. Given the feeble interaction of DMa with standard model fields, precise measurements are particularly promising for these searches. When one requires precision, a particular measurement stands out in astrophysics: the time of arrival (TOA) of radio waves from pulsars and fast radio bursts (FRBs). The use of pulsar timing has already been suggested to study the effects of dark matter [1][2][3][4][5][6][7][8][9]. In this Letter we present new results for DMa models directly coupled to light from the propagation of radio pulses from pulsars and FRBs. A more comprehensive exploration will be presented elsewhere [10].If DMa is coupled to the electromagnetic field, one expects modifications in the emission, propagation, and detection of radio pulses. We focus here on the effects during the propagation, which are robust under astrophysical uncertainties. In particular, we derive stringent constraints on millicharged DMa and axion-like particles (ALPs) based on dispersion measurements (DM) of radio signals from pulsars and FRBs, and on the modulation of the light polarization angle due to axion-like DMa in the Milky Way.We give a unified treatment, where the millicharged DMa and ALPs are considered as independent species. In the former case we consider that (a fraction of) the DMa is made of particles with mass m milli and electric charge q = e ( 1) [11][12][13][14][15][16][17][18]. As an example, this coupling arises in models where the DMa is charged under a dark photon, which is kinematically coupled to the visible photon [16,17]. In our analysis we remain agnostic to the origin of this term and other possible model-dependent signatures behind the charge of the DMa, and focus on constraining . Regarding ALPs, we assume the existence of axion-like [19][20][21][22][23], pseudo-scalar DMa of mass m a (represented by the field φ below).The relevant field equations readwhere g is the ALP-photon axial coupling, j ν is the ordinary electron current, whereas j ν milli is the current from millicharged particles. The role of this term in the propagation of radio-waves will...