Our aim is to precisely measure the physical parameters of the eclipsing binary IO Aqr and derive a distance to this system by applying a surface brightness - colour relation. Our motivation is to combine these parameters with future precise distance determinations from the GAIA space mission to derive precise surface brightness - colour relations for stars. We extensively used photometry from the Super-WASP and ASAS projects and precise radial velocities obtained from HARPS and CORALIE high-resolution spectra. We analysed light curves with the code JKTEBOP and radial velocity curves with the Wilson-Devinney program. We found that IO Aqr is a hierarchical triple system consisting of a double-lined short-period (P=2.37 d) spectroscopic binary and a low-luminosity and low-mass companion star orbiting the binary with a period of ~25000 d (~70 yr) on a very eccentric orbit. We derive high-precision (better than 1%) physical parameters of the inner binary, which is composed of two slightly evolved main-sequence stars (F5 V-IV + F6 V-IV) with masses of M1=1.569+/-0.004 and M2=1.655+/-0.004 M_sun and radii R1=2.19+/-0.02 and R2=2.49+/-0.02 R_sun. The companion is most probably a late K-type dwarf with mass ~0.6 M_sun. The distance to the system resulting from applying a (V-K) surface brightness - colour relation is 255+/-6(stat.)+/-6(sys.) pc, which agrees well with the Hipparcos value of 270+/-73 pc, but is more precise by a factor of eight.
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