The aim of this study was to test the ability of magnetic resonance imaging (MRI) technique to characterize gonad development and to determine the sex of live Pacific oysters through their shells. A preliminary nuclear magnetic resonance (NMR) relaxometry study was conducted to characterize T1 and T2 NMR relaxation parameters for the main oyster organs. This showed that T1-weighted MRI sequences were most appropriate to optimize contrasts between tissues in images. The results showed that gray levels of gonads in images acquired with gradient-echo sequence were variably affected by T2* weighting effect. However, the ovaries systematically gave a hypersignal in spin-echo T1-weighted images, and stack histograms of female oysters showed a peak well separated from that of male oysters. An automated method is proposed to quantify the development of oysters and their gonad maturation and to identify their sex.