Terahertz (THz) fields are widely used for sensing, communication and quality control(1). In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs)(2,3). These possibilities emerge from the strongly reduced GP wavelength, lambda(p), compared with the photon wavelength, lambda o, which can be controlled by modulating the carrier density of graphene via electrical gating(4-8). Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement(9-11), analogous to plasmons in twodimensional electron gases (2DEGs) near conductive substrates(12). Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM)(6,7), the real space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far only GPs with nearly free space wavelengths have been observed(13). Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically(14) rather than optically(6,7). This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (lambda(P) approximate to lambda(0)/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.