Spherical superparamagnetic iron oxide nanoparticles have been developed as T2-negative contrast agents for magnetic resonance imaging in clinical use because of their biocompatibility and ease of synthesis; however, they exhibit relatively low transverse relaxivity. Here we report a new strategy to achieve high transverse relaxivity by controlling the morphology of iron oxide nanoparticles. We successfully fabricate size-controllable octapod iron oxide nanoparticles by introducing chloride anions. The octapod iron oxide nanoparticles (edge length of 30 nm) exhibit an ultrahigh transverse relaxivity value (679.3 ± 30 mM(-1) s(-1)), indicating that these octapod iron oxide nanoparticles are much more effective T2 contrast agents for in vivo imaging and small tumour detection in comparison with conventional iron oxide nanoparticles, which holds great promise for highly sensitive, early stage and accurate detection of cancer in the clinic.