No treatment is available for patients with spinal cord injury (SCI). Patients often arrive to the hospital hours after SCI suggesting the need of a therapy that can be used on a clinically relevant window. Previous studies showed that Tamoxifen (TAM) treatment 24 h after SCI benefits locomotor recovery in female rats. Tamoxifen exerts beneficial effects in male and female rodents but a gap of knowledge exists on: the therapeutic window of TAM, the spatio-temporal mechanisms activated and if this response is sexually dimorphic. We hypothesized that TAM will favor locomotor recovery when administered up-to 24 h after SCI in male Sprague-Dawley rats. Rats received a thoracic (T10) contusion using the MACSIS impactor followed by placebo or TAM (15 mg/21 days) pellets in a therapeutic window of 0, 6, 12, or 24 h. Animals were sacrificed at 2, 7, 14, 28 or 35 days post injury (DPI) to study the molecular and cellular changes in the acute and chronic stages. Immediate or delayed therapy (t = 6 h) improved locomotor function, increased white matter spared tissue, and neuronal survival. TAM reduced reactive gliosis during chronic stages and increased the expression of Olig-2. A significant difference was observed in estrogen receptor alpha between male and female rodents from 2 to 28 DPI suggesting a sexually dimorphic characteristic that could be related to the behavioral differences observed in the therapeutic window of TAM. This study supports the use of TAM in the SCI setting due to its neuroprotective effects but with a significant sexually dimorphic therapeutic window.